Steel Inferno is a set of free wargames rules for modern armored warfare. The authors also provides statistics for some of Games Workshop’s Epic scale Warhammer 40k rules.
A part of the comprehensive PZ8 series, PZ8 1975 – 2010 is a two page set of free wargames rules for 6mm figures.
Mog ’93 is a set of free wargames rules for playing out the Blackhawk Down incident from Mogadishu 1993.
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.