Allegiance Source Code Released
Microsoft released the source to their game, Allegience. This is a 3D space conquest game that looks pretty complex - there's a pretty active community around the game, which influenced Microsoft's decision to post it (at Microsoft Research strangely enough). The whole thing - including the source to things like the game server's performance counter DLL, their Setup, editing tools etc...
And the code looks excellent - very clean, OO design.. check it out here.