Getaround is a brand new app launching today on the App Store that is designed to be a "peer-to-peer car rental marketplace." The idea is that it's like a Zipcar sort of service, but run peer-to-peer style, so anyone can rent their cars for just a few bucks an hour. If you need a car for a limited amount of time, you can load up the app, do a quick search in your area, and then find a car and an owner renting it just long enough to take a trip to the grocery store or pick up a piece of furniture.
And if you want to be an owner, you can sign up with the service online, and Getaround will cover the insurance after an eligibility screening, and even provides a special carkit that lets users unlock and lock their cars straight through the iPhone app (just like the official Zipcar app).
It seems like a great service, both for people who want to make a few bucks renting cars, and for current car sharing users looking for a cheaper or closer service than they're currently using. If you live in an urban area and could make use of a shared car, give it a look.