Stargrunt 40,000 is a system for converting Warhammer 40K army lists into a format usable with Stargrunt.
Rules: Games Workshop
Three Musketeers Legends of the Old West Variant
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.
Gemini Rules For Warhammer 40K
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 One Page Rules
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.
Age of Sigmar One Page Rules
The guys at One Page Rules have created a one-page version of Games Workshop’s Age of Sigmar.