Halogen, an unofficial fan project to create a real-time strategy game in the Halo universe, is shutting down due to a request from Microsoft. From their homepage:

Hours ago, we finally recieved the words we've been dreading since the mod started to get noticed. Microsoft has decided that we are infringing on the intellectual property of Bungie Studios and has asked us to stop development on Halogen.

The site notes that the forum and other areas will be shut down within a week.