http://www.getaround.com/
This certainly sounds like a neat start-up idea and seems to already have a strong following in Silicon Valley and San Francisco.
I used my buddy's address to search for cars and there are over 10 cars within several miles that are potentially available for rent. So, the interest is definitely there.
You rent by the hour on terms negotiated between you and the vehicle owner, but getaround.com pays to insure the vehicle for the duration of the rental. Of course, the issue of minor damage during rental period and whether the vehicle will be used for illegal activities is going to be a concern. Obviously this type of service will require a lot of due diligence from both parties, but it could work.
Although it is unlikely that any BITOGer or car enthusiast will participate in this type of program, I can see this becoming a less expensive and more flexible alternative to traditional car rental programs in urban areas. More of the kinks need to be worked out but the idea is solid.