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!
They've posted several videos to YouTube that show timelapsed edits to maps; here's Islamabad, Pakistan coming into existence. (via o'reilly radar)
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)