I've been a member of Zipcar for a couple years now, but I've been using it a bit more recently, which has confirmed how awesome it is. Having it available makes it much more feasible to not own a car in cities (all American ones except NYC, basically) where you do need one occasionally. It would be a huge hassle to do a normal car rental to run an errand to the suburbs, and very expensive to take a cab, but Zipcar lets me do it inexpensively on a moment's notice. It was even slightly cheaper than renting a car for three days for my recent Catskills trip (though admitttedly this is mostly because the rentals were unusually expensive), and far more pleasant because we didn't have to trek down to the rental car office, fill out the paperwork, and turn down all the extras they would have tried to sell us.
And Zipcar has just released an iPhone app that does snazzy things like let you find out which cars are available near wherever you happen to be. You can also lock and unlock the car from your phone, as well as honk the horn.
There are Zipcars in Pittsburgh. If, after arriving there on my bike next week, I decide I want to get somewhere that's hard to bike to, I can check my phone for the nearest cars, reserve one with a few taps, and drive away. Pretty slick.