A Splendid Victory is a set of free wargames rules for the English Civil War. They’re designed for 28mm figures, but the basing system is flexible.
Rules: Pike and Shot
Richard Bradley’s Samurai Skirmish Rules
The prolific Richard Bradley has a set of free wargames rules for samurai skirmishes.
Richard Bradley’s Pike and Shot Skirmish Rules
Richard Bradley offers a set of Pike and Shot Skirmish rules.
Renaissance Naval Rules
David Heading offers a set of free wargames rules for Renaissance Naval Battles.
Victory Without Squares
Victory without Squares is a modification of the Victory without Quarter English Civil War Rules System. Both are free wargames rules, with the “without squares” version using inches, rather than squares for movement and range.