open street maps
Take a look at www.openstreetmap.org. It is a wiki-like thing for editing street maps. I liked it a lot, and ended up "mapping" parts of Prague I know and I could recognize from the sattelite pictures.
It has some problems -- java interface for editing is ugly, I'm not sure if one street can consist of different segments, no support for one way streets, and I'm not sure how intersections are handled -- but the interesting thing is that it actually looks feasible. After all, it is much less work to map a street than to actually build it :-).
[Corrected typo, thanks, anonymous!]