Google is soliciting contributions to Google Maps with their Map Maker service.
With Google Map Maker, you can become a citizen cartographer and help improve the quality of maps and local information in your region. You are invited to map the world with us!
Update: Several people wrote in to recommend OpenStreetMap instead because Google doesn’t make the data available in a raw form whereas the OSM data is under a CC license available for derivative works like OpenCycleMap. (thx, mike and everyone)