Mesh Networks
I saw an article about this a little while ago.  If anywhere in the world could support a wide scale mesh network, it would have to be the Bay Area.  It would be a really cool experiment to blanket a part of the Valley in mesh wifi: I imagine it would be very doable to raise $100K and send 1,500 mesh routers to people in Palo Alto or SOMA.  Open Mesh has some really cool low-cost ($50 - $75) hardware that seems to just work: http://www.open-mesh.com/.  Some might be plugged into an upstream link, but if most were only powered on as relays it would still work.