The OpenStreetMap wiki is a very active wiki supporting the OpenStreetMap project. The wiki has project documentation for the main OpenStreetMap server software and also for the myriad of other open source offerings in and around the project, and branching out to document many geo-technologies in general (but mainly from an OpenStreetMap perspective)
OpenStreetMap itself is a project to do with maps, but interestingly it is applying wiki principles to map making. Contributors can view the map and then move across to the edit tab and edit the map! The project is modelled on wikipedia, and like wikipedia, is overseen by a not-for-profit foundation, and releases all of this map data with an open license. With > 300,000 registered users it's a very active community of map makers.