In Montreal, we participated in a carshare called Communauto. The goal of Communauto is to have a car at least every 500 metres across the island of Montreal. In practice for areas they serve, they usually had more than that. The Saint-Sacrement location had at least a dozen, and there were clusters of 3 or 4 within a couple minutes walking distance of anywhere on the plateau.
Such a thing does exist here, but not in Silicon Valley at all. If you're in the valley at all and wish you didn't need to have a car for little errands, or are thinking about a second car for those times you just want to go somewhere different from your mate, *please* add your name to the notify list for zipcar:
http://www.zipcar.com/notify-meThe problem this solves is the one that a bus or a bike can get one to work easily enough, or into the city, but the bus is never going to be great for the once-a-week grocery shop, or that trip to Ikea. In that case, you pay the use of the car for an hour (or 4 if it's Ikea...), and then return it. I really believe that it's a critical part of making mass-transit work.