Bad AR:SE is a set of free wargames rules for modern skirmishes.
Ty Beard offers an introductory set of his Fistful of Tows rules for modern warfare.
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.