Car2go Smart fortwo and iPad app
Car2Go is, basically, a Zipcar competitor. Like Zipcar you get an RFID card that provides access. Just walk up, tap it on the windshield, and the car unlocks. Right now you find the keys hiding in the glove box and, after punching in a PIN and accepting a few terms and conditions on the car's center-mounted touchscreen, you're ready to roll. But, there are some differences between the services, like no annual fee with Car2go, offset by a higher one-time registration, $35 vs. $25 for Zip, and a potentially higher hourly fee as well, at $12.99 maximum per hour vs. $8.50 (though Zip's fee does go up to $13.25 on weekends).
And that leads to one of the big changes with Car2go versus the others: you can park anywhere you like. Well, anywhere that's legal, anyway. Car location is updated via GPS and monitored centrally, along with information like fuel level and plenty of other metrics, all uploaded over an integrated data connection. It's T-Mobile. Using that connection you can also request that the car get a cleaning before it's rented again, and even call their support center -- hands-free, of course.
In practice the process is easy enough, though it's the cars that may leave some wishing for a bit more. If you've never been in a Fortwo they are perhaps a bit more comfortable than you might have expected, but they're certainly no larger than you'd hoped. There's plenty of storage in the back for a messenger bag and a jacket but not much more. You could certainly stuff a couple loads of laundry in there if you don't care about how wrinkled they'd get on the way home, but making a run to Ikea to pick up a new and exotically named home entertainment center is definitely not in the cards.
Still, it's a great little car for short intra-city missions, and the service is structured to match that. The biggest problem is, of course, availability, and 210 cars in one American city (there are two more in Germany) are hardly going to revolutionize modern transportation. The company promises us that "a couple" of new cities are coming "within the next year" and, while that's too vague for our tastes, it's certainly a lot better than "nowhere" and "never."
Update: A couple commenters noted that there's an Android app called Find2Car that works well in Germany, but as of now is not available for booking cars based in Austin.