Imagine Image offers the freeHex Command Ancients Rules for miniatures wargames. The hex system is supposed to simplify things and speed play. You can play without a hex board if you want.
Miniatures Rules
Little Lambent Meteors
Jim Wallman’s Little Lambent Meteors is a set of free wargames rules for 18th Century riots, using 15mm figures.
Road Warriors rules
Michael Taylor has written Road Warriors, a set of free wargames rules that uses Matchbox cars for games set in a post-apocalyptic future, ala Mad Max.
1 Page Napoleonic Skirmish
The Jackson Gamers have these free wargames rules for Napoleonic skirmishes. The best part: the rules are just one page long.