Ravenfeast is a set of free wargames rules for skirmishes in the Norse sagas.
Miniatures Rules
Brutality Skirmish Wargame
Brutality is a miniatures agnostic skirmish wargame set in a science fantasy realm. You can get a free version at the link.
The AD Wars Science Fiction Rules
The AD Wars is a set of free wargames rules that are adaptation of Warhammer Ancient Battles for science fiction games. It’s interesting. You will need a copy of Warhammer Ancients to play, though. But there are a lot of copies of that out there.
link
Sixties Spy Skirmish Rules
Sixties Spy Skirmish Rules are a set of free wargames rules for James Bond style games.
Napoleonic Campaign Rules
Dale the Wargamer offers a set of Napoleonic Campaign Rules to be played on a Risk board.
Link.