The Open Street Map project uses Wiki principles to collectively build a map. Contributors gather information by walking, driving or cycling the area; from GPS traces, out-of-copyright maps, satellite imagery, and the like.
It lacks the detailed satellite imagery and route-finding capabilities of commercial products, and some areas look a bit thin, but elsewhere, including this part of the world, the results are already pretty impressive. Places like central London even more so, and things seem to be moving forward pretty quickly.
Even better: because it is all based on an open philosophy, there is tremendous potential for specialised spin-offs. For example, Andy Allan has produced a version designed specifically for cyclists, which is here, and there is also a London tube map by Steve Chilton.