Have you checked out Wikimapia? It’s an open source Google Maps mashup.
You can just browse what people have labeled, you can search the map and even add to it. Check out what’s been going on with the labeling of Los Angeles.
Some of the labels are obvious things, like landmarks and interchanges, but others are fascinating little tidbits like a stretch of Western in San Pedro that someone seems to think folks drive to fast on.
The only thing that I don’t like about it so far is that you can’t draw irregular shapes. Everything is rectangular. I’d like to be able to put legislative, district and council borders on there, but it’s just not possible at the moment. How cool would that be? You just find your house and you can see at a glance where your police station is, who your city councilperson is, who your state rep is and so on.