Refers to the joint ownership of a car, not to sharing a ride such as in Carpooling
Wikipedia article is at http://en.wikipedia.org/wiki/Car_sharing
See also more specifically: P2P Carsharing
"Car Sharing Is the Gateway. Car-sharing is the gateway drug of the growing trend, and according to a report from research firm Latitude called The Sharing Economy, people who try out car-sharing services are more likely to join in other web-based sharing services. Car sharers also share significantly more across categories than non-car sharers." 
"Carsharing is a system in which a fleet of cars is jointly-owned by the users.
This is different from Car pooling in which cars are owned privately and simply drivers allows passengers to use the empty car sits on their car. The key difference between Carsharing and Carpooling: carpooling exploits the cars already owned by people, carsharing exploits cars bought precisely for and by the carsharing group." (http://wiki.couchsurfing.com/en/CarSharing)
P2P Carsharing: Consumer to Consumer
"A fleet of cars is owned by a community. The marketplace matches owners of cars that are available to other drivers to rent."
Business to Consumer
A company owns a fleet of cars and facilitates the sharing amongst members.
- AUTO MANUFACTURERS: BMW, Peugeot, Daimler
- RENTAL BRANDS: Hertz, WeCar
- CAR SHARING BRANDS: Zipcar, StattAuto, GoGet
Nonprofit Cooperatives (or Public Initiatives)
A local organization or community that facilitates car sharing with the goal of changing driving habits over making a profit.
- City Car Share
- I-GO Chicago
Or: the publicly owned Autolib in France 
Statistics and Economic Value
"Car Sharing pilot projects like Witkar began as early as the 1960's and 1970's, but modern car sharing programs launched in 1987 in Switzerland and later in 1988 in Germany, and came to North America via Quebec City in 1994. [The first successful car share in the USA, Portland Oregon's CarSharing-PDX
As of July 1, 2011- based on data provided by Susan Shaheen, University of California, Berkeley - 26 U.S. carsharing programs claimed 560,572 members sharing 10,019 vehicles; and 78,840 members shared 2,605 vehicles among 20 carsharing organizations in Canada." (http://www.carsharing.net/)
- "Carsharing revenues alone are projected to hit $3.3 billion by 2016, according to business research and consulting firm Frost & Sullivan." (http://knowledge.wharton.upenn.edu/arabic/article.cfm?articleid=2714)
- Sustainability effect: IN 2009, CAR SHARING DIMINISHED GLOBAL CARBON DIOXIDE EMISSIONS BY 482,170 Tons 
- Tipping Point: In 2009, for the first time, the number of Americans who ditched their cars was greater than those who purchased
new cars. 10b bought cars, 14b sold their cars. (Fast Company via )
- Top 3 reasons for carsharing: 1) cost savings; 2) convenience and location; 3) guaranteed parking space (Source: CarSharing.net)
- The average car is used one hour per day; average car costs $715 per month (U.S.) (RelayRides)
- Over the last 20 years, Germany has experienced a sharp decrease in new car buyers between 18 and 29 years old. From in 1998, 16.4% of new car buyers
were 18 - 29 y.o. to in 2011, 7.6% of new car buyers were 18 - 29 y.o. (Deutsche Welle)
- FOR EVERY RENTED CAR, THERE ARE 15 FEWER OWNED CARS ON THE ROAD. (Zipcar)
- CARSHARING MEMBERS DROVE 31% Less THAN WHEN THEY OWNED A PERSONAL VEHICLE (Zipcar)
The weakness of current carsharing models
By Erica Barnett at http://www.worldchanging.com/archives/007087.html :
"My only real complaint about carsharing is that it has one major, built-in, self-limiting inefficiency: because cars must be returned to the spot where you pick them up, you can't take one-way trips. That means that even if what I really want to do is drive from my house to the store and to a friend's party, from which I'll get a ride or return by bus, I'm forced to drive to the party and drive home. (This also automatically makes me the designated driver.)
What's the solution to this conundrum? Actually, it's intuitive: supply, supply, supply. The more cars there are, the easier they are to access and use. So it stands to reason that when the ratio of cars to drivers reaches a certain saturation point, it will become possible to take a car from one carsharing spot and return it to another. Perhaps this will mean moving, at least initially, toward a model in which individual spots around a city are centralized into a smaller number of carsharing stations of half a dozen or more cars each. (In Seattle, these are called "pods.") Drivers could take whatever car is available, with specific cars such as trucks or convertibles available on a more limited basis for a premium.
Neither of the two largest carsharing companies in the U.S., FlexCar and ZipCar, currently offer one-way trips. However, my prediction is that as the popularity of carsharing increases and the companies' fleets grow larger, one-way trips are inevitable. And it's likely that this will only make carsharing more appealing: since 2000, Berlin's GreenWheels has allowed one-way driving, resulting in a 23 percent increase in trips.
Cities could take proactive steps to make one-way come sooner. Most carsharing programs are public-private partnerships already. A simple carbon tax on vehicle emissions -- charged either per ton of emissions produced, or as a flat motor-vehicle tax when drivers renews their registration -- could feed into a grant program for carsharing expansion, which would in time fund the purchase of enough cars to make one-way driving viable. New cars and one-way possibilities would, in turn, increase the visibility and popularity of carsharing, helping to make it a viable alternative for those who currently consider it an impractical luxury."
Car-sharing with a Small Group
"Some solutions are designed for ready-made networks of people. In their report Collaborative Services: Social Innovation and Design for Sustainability, François Jégou and Ezio Manzini discuss how residents of multi-unit buildings could create a group car-share account to ensure that they have access to specific vehicles nearby or in their building's lot:
- Using existing car-sharing businesses to set up a group car-share account for a building — residents are not obligated to join but having such an account will ensure that there are several available vehicles in a building’s lot or very close to it. The company can adjust how many vehicles it makes available to the building based on demand, and when the cars are not being used by residents, they are available to other members, thereby helping to keep the whole endeavor profitable. “It’s like classic car-sharing, but with one or several parking spaces near your building. We benefit from professionally managed vehicles: reservations via mobile telephone, maintenance, registration and replacement of vehicles — all without having to organize it." (http://www.worldchanging.com/archives/009760.html)
1. Beth Buczynski:
“a recent study by the University of California at Berkeley found that saving money isn't the only compelling reason to support car sharing initiatives.
An online survey of over 9,500 individuals living in Canada and the United States revealed that car sharing programs have the power to significantly reduce overall greenhouse gas emissions in North America.
The survey, which polled members of major car sharing organizations, found that for most car sharing members, individual greenhouse gas emissions increased because the program provided access to a personal vehicle for the first time. However, these small individual increases were outweighed by the significant number of people able to drastically reduce their emissions through car sharing. "The number of carless households increasing their emissions is comparatively large, constituting more than half of the respondents," noted the researches. "However, the degree to which these households are increasing emissions as a result of carsharing is small on an individual basis. The overall emission reduction is driven by the remaining respondents reducing their emissions by larger amounts that compensate for increases of the majority."
Among multivehicle households shedding cars, 88% of respondents reduced emissions. Similarly, among single-vehicle households shedding cars, 93% exhibited an emission reduction.
So where's this impact coming from?
In general, the researchers found that participating in a car sharing program helps members to embrace a low-mileage lifestyle, even though carless households converge to this lifestyle by increasing emissions, and car-holding households converge by going car free and decreasing emissions.” (http://www.shareable.net/blog/car-sharing-antidote-to-ghg-emissions-in-north-america)
2. By Jessica Scorpio:
"The environmental benefits of car sharing are clear. In North America, one shared vehicle can take 6 to 33 privately owned vehicles off the road. Lowering vehicle ownership reduces the amount that people drive. In one study, vehicle miles traveled by car-sharing members dropped by 67 percent while they were engaged in a car-sharing program. They were able to maintain their standard of mobility and their convenience of transportation (two success criteria for car sharing in general), while actually getting access to a wider variety of cars.
The dramatic drop in vehicle miles traveled has an immense environmental impact – one study shows car sharing resulted in a decrease in greenhouse gas emissions ranging from 39 to 64 percent." (http://shareable.net/blog/share-your-car-join-the-new-economy)
- Zipcar ;
- I Go
- My City Wheels
- Greenwheels in the Netherlands
- rent'n'roll in Germany
"For drivers who already share movies via Netflix and stream music rather than buying CDs, the idea of sharing a car is the natural extension of a hip, financially smart, and environmentally conscious urban lifestyle.
After all, drivers who give up their cars and switch to Zipcar say they save an average of $600 per month. Car sharers report reducing their vehicle miles traveled by 44%, according to Susan Shaheen of the University of California at Berkeley, and surveys in Europe show CO2 emissions are being cut by up to 50% per user.
"When I meet another Zipcar member at a party or something, I feel like we have something in common," says Francis Smith, a photographer who lives in Brooklyn. "It's like we're both making intelligent choices about our lives."
Businesses are catching on too. About 8,500 companies have signed up for the service, including Lockheed Martin (LMT, Fortune 500), Gap (GPS, Fortune 500), and Nike (NKE, Fortune 500). So have 120 colleges and universities, such as Carnegie Mellon and the University of Miami.
Zipcar is also marketing its technology -- the hardware and software that keep track of the cars -- to city governments. Washington, D.C., retrofitted its fleet this year using Zipcar's wireless systems and estimates that the move will save it $1 million a year. "It's just such a no-brainer," says Ralph Burns, who manages the D.C. government fleet. "Agencies putting their budgets together for next year are calling me up and saying, 'Ralph, I've got 25 cars I want to get rid of!'"
Though car sharing is an audacious challenge to the whole principle of car ownership -- each shared vehicle takes up to 20 cars off the road as members sell their rides or decide not to buy new ones, says Shaheen -- the auto industry is increasingly realizing it has little choice but to play ball.
Toyota (TM) and Ford (F, Fortune 500) have already begun exploring ways to work with Zipcar, from using its members to test electric cars to designing vehicles specifically for the sharing market." (http://money.cnn.com/2009/08/26/news/companies/zipcar_car_rentals.fortune/)
A fleet by the people and for the people. A revolutionary way to share values underutilized assets with your neighbor or your community. They build a trusted community by doing extensive and on going background checks for members and installing a custom built device in the owner's shared asset, which in turn helps people share their assets with full trust and security. Rather than buying new cars/assets, they promote owners to share their used underutilized car/asset among their community enabling them to make money. By sharing just less than 4 days a week the owner would be able to pay the maintenance cost or monthly loan payment of their car still have money to spare.
Chevron said "that by 2030, there will be 50% more vehicles in the world", so JustShareIt conducted a survey and found that most of the people are likely to share but would not because of safety, trust and security. To eliminate trust, security and safety they created their software and hardware which helps them to facilitate all their transactions and provide ease of mind to the owners and borrowers. They believe this is a new way to have your depreciated asset make money for you. "Car sharing and other highly priced underutilized asset sharing is the first step in the right direction to save environment and bring people closer together" says JustShareIt. They believe that everyone in the same community does not need to own a boat or an SUV etc., people are most likely to share their car/assets with others who are part of the same family vs strangers. They eliminate key exchanges between the owner & borrower and through their technology they are able to control the transaction in a trusted and secured manner. "What is mine is not mine it's ours" is their moto.
France: Autolib in Paris
"The results are in from the first few months of Paris’s revolutionary municipal car sharing program. And while it already has 6,000 subscribers, a large number of cars have been vandalized or broken down." 
"The new Autolib' program (a play on liberté, the the French word for liberty) launched on Sunday. It is a public car scheme expected to grow from its current 66 vehicles to 250 by December, and then to more than 3,000 vehicles by 2012 in the French capital and surrounding cities. Billionaire investor Vincent Bolloré teamed up with Bertrand Delanoë, the socialist mayor of Paris, to launch the venture.
Today, if you fly into the City of Light, you can already pedal around the city car (and care) free after paying a small Velib' membership fee. Short bicycle trips are free, and the 20,000 bikes are taken on millions of trips each year. The city's shared bike system even inspired similar program here in the U.S., such as CapitalBikeshare installed in Washington D.C., and New York's soon-to-launch program.
But such city-run car-sharing options have failed to materialize as fast as bike sharing, for obvious reasons. Autolib' estimates it will need at least 80,000 users to turn a profit. At about $12 per hour, and a $189 annual fee, it's still a bargain in Europe where private transport and fuel are much more expensive than in the U.S.
While the U.S. has yet to see a municipally run car sharing program, nonprofit groups like the San Francisco group City Car Share, Chicago's I-Go Carsharing, and Boulder's e-Go Carshare have gotten into the game mostly dominated by the for-profit Zipcar." (http://www.fastcoexist.com/1678604/autolib-could-pariss-electric-car-share-be-a-test-case-for-cities-around-the-world)
- World Cities Carshare Inventory at http://ecoplan.org/carshare/general/cities.htm
- CarSharing Association
- World CarShare forum: Exceptional information aimed at busy carshare practitioners 
- Carshare Cafe - A wide open chat space for exchanges of broad general interest on car sharing 
- World Carshare Consortium 
- DIY Car Sharing: How to Start Your Own Car Sharing Program
- How To Share a Car With A Stranger 
More and updates via http://www.carsharing.net/library/index.html
- Car-Sharing: Where and How it Succeeds 
TRB's Transit Co-operative Research Program (Dec 2005)
- Car Sharing reduces driving by 47% 
UC Berkley Study of City CarShare, Jan 2004
- CarSharing - Start Up Issues and New Operational Models 
David Brook; January 2004
- The Beginner's Guide to the Car Sharing Business 
For people who want to bring car sharing to their community.
- Thesis about P2P carsharing - Aaron Lewis & Mark Simmons (2012); http://p2pcarsharing.us.com/
- Thesis on P2P carsharing - Jessica Abad Kelly (2011); Contact http://about.me/jabadkelly for thesis access
* Robin Chase, CEO de Buzzcar.com, * Frédéric Mazzella, CEO de Covoiturage.fr * Jean-Louis Jourdan, Directeur du développement durable de la SNCF