The Fimir Army Book is the latest in a series of high quality fan-created Army books for Warhammer Fantasy.
Game Workshop’s Space Hulk game is one of the best it’s every produced. Every game is fraught with tension, as the marines attempt to maximize their scarce action points in the face of an increasing onslaught of genestealers. With good players, this game is nearly always a near run thing. Unfortunately, like so many of their efforts, the Evil Empire hasn’t continued to support it. Expanding The Hulk is a website dedicated to continuing the Space Hulk game. It’s got rules for using many of the other races in the Games Workshop universe, including Chaos, Eldar, Dark Eldar, Necrons, and Orcs.
Stargrunt 40,000 is a system for converting Warhammer 40K army lists into a format usable with Stargrunt.
The Cops.NU site has a Warhammer 40K variant for the American Civil War. I think it could work.
Sgt. Perry’s Heroes offers a variant of the Legends of the Old West rules for miniature wargames set in the era of the Three Musketeers.