When I was still studying cartography at uni I came across Wikimapia, which was then more or less starting. I totally loved it.
It doesn’t really have the depth of wikipedia, you won’t find info about the Taylor Spatial Frame, the list of rice dishes, or how you can show respect to Quebecers like on wikipedia. However, for me, it is an amazing way of travelling. I once ‘visited’ all the islands in the Caribeans. Hoping from one island to the other, I discovered that Saint Lucia was the country with the highest proportion of Nobel Prize winners. How cool! More recently, I tested the moderation system and added the ‘Bermuda Triangle‘ in Copenhagen. That’s what I call it, and I wonder how long it will take before people start calling it that way. Wikimapia, which has its own Wikipedia entry, is not part of the Wikipedia project.
More recently I found the project ‘Mapping wikipedia‘ from TraceMedia and Oxford Internet Institute, which maps wikipedia entries. Unfortunately, it does not use polygons, only points. This makes it difficult to get a quick overview of the relative spatial importance of each entry, which I think is very powerful. A shame, but maybe the project is meant to grow?