Sixties Spy Skirmish Rules are a set of free wargames rules for James Bond style games.
Fubar is a good looking one page set of modern and science fiction rules.
Aqua Zone is a set of free wargames rules for playing miniatures games that feature underwater combat with submarines, DSV’s and marine animals.
Threat is a modern micro armor game. The authors write:
THREAT is a modern-era miniatures wargaming ruleset that was designed to be a fast-moving fluid game of modern tactical ground combat using 1/285 scale miniatures.
THREAT is a very realistic simulation of the deadly modern battlefield. Combat is simultaneous, for this reason a referee is recommended to direct resolution of the firing and resolve line of sight (LOS) problems. Each miniature represents a one-for-one ratio of personnel and equipment. Aircraft may represent multiples of aircraft of similar type.
Unlike other game rules, THREAT does not use individual vehicle charts, rather it uses charts for each weapon system that modern vehicles employ. In this way any number of vehicles with different weapons mountings can be utilizied without having to create new charts.
GHQ has a free pdf of its Beer and Pretzels Modern Micro Armor game, formerly a commercial effort, now a set of free wargames rules.