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
Zombie Squad RPG
Zombie Squad is billed as a “zombie horror survival” RPG. Looks like fun.
Sixties Spy Skirmish Rules
Sixties Spy Skirmish Rules are a set of free wargames rules for James Bond style games.
Dexterity Jane Print and Play Game
Dexterity Jane is an interesting party game that involves making hand signals to reflect the prompts on cards on the table. It will be available in the fall as a commercial product, but you can get a print-and play at the link: