Tabletop Titans is a set of free wargames rules for superhero miniatures games.
Boonta Eve Classic is a set of free wargmes rules for pod racing in the Star Wars Universe. The rules are designed for use with those old Galoob Micromachines, which can still be had for a reasonable price on Ebay:
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Firefight Mass Combat is a set of free wargames rules originally written for use with Alternative Armies' 15mm science fiction ranges.
Cars! is a set of free wargames rules for auto combat.