Being a fan of the Socialzed Umbrella Theory (like a lending library for umbrellas - take one if you need one), this PhillyCarShare idea really hits home. They ask 'What is Car Sharing?" on the site and offer this answer:
Car sharing is a transportation service that makes vehicles available to people on a per-use basis. Think of it as a neighborhood-based time-share that allows you to use vehicles when you want to, and pay for only how much you drive. Car sharing provides a way to dramatically reduce the number of cars in an urban area and is a practical way to support your neighborhood and protect the natural environment.
Car sharing is a great, and green, idea. There have been some social experiements with non-commercial applications in case you're interested. An MIT PhdD candidate did a review of bicycle sharing. Bookcrossing took book sharing to a new level, with people leaving books to be discovered by others. And we all know that photo sharing is all the rage. I'll check out PhillyCarShare myself and give a first person report in the next week or two.
- Update: I'm a little embarrassed. I see that Howard had posted on this down below. My apologies. I need to get over this reading ADD.

Philly Car Share
I've been doing it for about a month and have loved it. We've already cut back on the use of our old car and are going to sell it. Philly Car Share saves money for anyone who drives less than 10,000 miles/year. And you get to drive hybrids. If you are doing it, though, PM me and I'll give you my name so that we can each get a referral award.
Will do
I am going to sign up today or tomorrow, and saw they have a very good referral system. I'll send you a note.
Glad that's working. It really seems a great idea on second and third reading, even better that it works in practice. Besides, I want to drive a Prius anyways (was looking at buying one, but now don't have to).
i always see these cars around
i think that i may join this program soon as well. share with us your experiences!