Roar of the Crowd is a set of free rules for playing science fiction, fantasy or historical gladiator games.
science fiction
Dr. Who Miniatures Rules
Here’s a link to a set of Dr. Who miniatures rules.
Astral Dreams Space Opera Miniatures Game
Astral Dreams is a free miniatures game based on the d20 Open Gaming License system. The game’s background is based on a short story of the same name, which involves a war between humans and robots.
GrimDark Future Rules v. 2.3
The folk at One Page Rules have released a new edition of GrimDark Future, a set of one-page rules (with lots of pages of armies you can also download)
Under The Broken Moon Role Playing Game
Under the Broken Moon is an RPG based on the Thundarr the Barbarian cartoon series. The game uses a mechanic where you roll a number of dice equal to your trait score and sum the results.