Seas of Lead is a set of free wargames rules for World War II Naval games. Along with the rules are some good looking unit data cards that they made using the Magic Set Editor.
Miniatures Rules
Red Hordes Russian Civil War Rules
Red Hordes is a set of free wargames rules for the Russian Vivil War. They’re similar to DBA/DBM.
Sandokan: The Malayan Tiger Colonial Rules
Sandokan: The Malayan Tiger is a set of free wargames rules for fast play colonial wargames set in Asia.
Star Wars Shoot Out
Star Wars Shoot Out is a set of free wargames rules for skirmishes in the Star Wars Universe.
Fast Play Jet Combat
Fast Play Jet Combat is a set of free wargames rules for 1/600 aircraft on a hex grid. Interestingly, it requires a d100 die. If you don’t have one handy (and who does), you can do the same thing with two d10s in different colors.