Interestingly, Microsoft has a product called TFS that has a great wiki implementation: it's native markdown (with github enhancements), a decent in-browser editor/viewer, and it's backed by git. Plus it has some nice features that let you refer to people with the at-symbol (@) and work items (bugs, tasks, stories, features, etc) with a hash symbol (#), and some other stuff. Github itself has some of these features; I'm not aware of anything else like it (Tiddlywiki, ignoring it's unusual runtime, may be the closest that I'm aware of).