Black Hat, White Hat is a set of free wargames rules for Wild West skirmishes.
The Shining Print and Play Game
The Shining is a print-and-play game based on the Stephen King novel of the same name. King himself apparently participated in playtesting.
Universal Combat System
Universal Combat System is a set of free wargame srules described by the authors of being for “any number of players in an infinite amount of settings.”
City of Mist RPG Starter Set
You can get a free downloadable City of Mist RPG Starter Set. City of Mist has an interesting premise. It’s a noir type setting in which you play a character that is the incarnation of mythic and literary characters. You could be King Arthur, for example, or Little Red Riding Hood.
Dead Simple Steampunk RPG
Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.