After being carless for about 2 1/2 years the Atrios household signed up for Philly Car Share. For $15/month per household membership fee you get access to a bunch of cars parked in various spots around town. There's an online reservation system and once you've reserved you just go pick your car at the correct time. There's no paperwork or anything when you do so, you just unlock the car with the electronic gizmo they give you and drive away. Cars are mostly hybrid vehicles which are interesting to drive. You pay $5.90 per hour for the time you reserved, and an additional 9 cents per mile. Gas, insurance, etc.. are included in that. Overnight and daily rates are cheaper.
We've only used it a couple of times so far but we've been quite happy. It's much more convenient than normal car rental, and not really any more expensive (yes you can get bargain daily rentals frequently but if you don't own a car you don't have any insurance so you end up paying the car rental insurance rates on top of that). Useful for those occasional trips to the burbs or to anywhere a cab or public transit isn't a good option.