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.
The Gemini rules for Warhammer 40,000 are essentially a set of house rules for Games Workshop’s flagship product.
I think that the author is correct in his premise — that the Rogue Trader rules were perhaps the best.
GrimDark Racing is a set of simple rules for races in the Warhammer 40K universe. Just use models you already have. There’s plenty of violence, of course.
The guys at One Page Rules have created a one-page version of Games Workshop’s Age of Sigmar.