RelayRides

I’d been car-less in SF for about a year. I hadn‘t signed up for Zipcar or City Carshare because I didn’t want to have to give money up-front when I wasn’t sure if/when I might use a car. And their various plans (with permutations re: mileage, time, gas) seemed so complicated. Then I saw a blue car w/ a RelayRides sticker a block from my home. I was surprised at how simple it was: no up-front fees; gas included; hourly rates. The registration & approval process was quick. For my 1st trip, I reserved the blue car online 1/2-hour beforehand, and everything went smoothly.
It’s cool that car owners get some benefit instead of having their cars sit idle all day. There's a wide variety of cars; I’ve rented hatchbacks for hauling furniture. If you don’t mind driving an older-model with maybe faded paint, you'll save a lot compared to Zipcar—which to me is definitely worth it.
RR has (nice) service reps answering the phone 24/7, too.