Today I came across mapping wiki style. OpenStreetMap aims to provide free geographic data of the whole world.
The current problems with most maps is that they are have copyright on them, and they have special easter eggs or gotchas to determine copying of the maps. This is a problem for projects that need to be able to use maps in the form of free speech, rather than free beer. Google Maps for example can be used like free beer, but not in the form of free speech.
The amount of data available is currently fairly low, so I would start using the maps for major map planning yet. However you can help, by mapping some part of the world that you know well.
I have recently been using MapMyRide.com for mapping out my Cycle trips. It uses Google Maps, which unfortunately does not contain enough information like the canals for example. This makes it difficult to do some mapping, especially in areas where their maps are of low resolution.