Brian Scates, creative director at a Silicon Valley startup, rented out his 2000 Audi All-Road last year for $50 to $60 a day, rather than let it sit unused while he biked around town and commuted to work by train. Meanwhile, Sebastien Rouif throws his surfboard into the back of his neighbor's pickup truck on Saturdays to drive down the coast and catch some waves. The fee and gas total about $40, cheaper than other rental options—and it's a lot less expensive than owning a car. (Related Pictures: "Twelve Car-Free City Zones")

Scates has since cut back on sharing his car, but still believes in the idea: "I'm all about fewer cars on the road, and maximizing the value that