I've heard of OpenStreetMap just one or two weeks ago. This is what I was looking for some years ago, but didn't find it back then.

Mainly I'm interested in writing Software which operates on GIS-Data, I haven't the time to do this right now.

But I had some GPX-Tracks which I recorded with my Sony Erricson V600i some months ago, and used them to map parts of my home town Wernigerode. It's kind of addictive.