Les Trois Mosquetaires is a set of free wargames rules for skirmishes in the world of Alexandre Dumas’ Three Musketeers
Miniatures Rules
Knights of the Sky WWI Aircraft Rules
Knights of the Sky is a set of free wargames rules for WWI Aircraft.
Warpath Science Fiction Rules From Mantic
Mantic offers a free download of its Warpath Science fiction rules.
Free Dreadball Rules From Mantic
Mantic offers a free download of its Dreadball rules for science fiction sports / combat.
Wargames Research Group Ancients – 5th Edition
Wargames Research Group has a page with lots of their older sets available, including the Fifth Edition Ancients, which were the standard for many years.