Core Game Development Studios offers a set of free wargames rules called Timeline. From the author: Timeline Skirmish is the skirmish game covering all historical ages and into the far future. This game is meant to be fast and furious.
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Dunger Racing Game
Dunger Derby is a miniatures game of racing little wasteland beasties called Dungers. You really have to take a look to get it.
Warlord 2085 Science Fiction Rules
Skank Games offers Warlord 2085, a set of free wargames rules for fast play science fiction miniatures.
Strange Aeons 18th Century Modifications
Strange Aeons is a commerical rules set, but if you already own it, you may be interested in these free modifications for playing in an 18th Century setting.
A Splendid Victory ECW Rules
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.