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.
Combined Arms 2000 is a free print-and-play board game for modern era combat. The description from Board Game Geek:
Combined Arms 2000 is a light wargame simulating armored warfare using modern technology. It features a realistic scale for both movement and fire (1 space = 1 kilometer), a good selection of vehicle and infantry types, off-board fire support, and fixed-wing air support.
Players build armies from parts of a shared deck, then fight! The attacker has only ten turns to seize his objectives. Wise use of reserves, clever scouting, and bold maneuver are all needed to attain victory.
This is a free print & play release on BoardGameGeek. Print cards and box to 110 lb cardstock, and rules to a single sheet of typing paper. Heavy lines on the box are cut lines; lighter lines are all mountain folds.
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.