Caveman Clash is a set of free wargames rules for playing skirmishes between cavemen. There also are paper figures if you don’t happen to have cavemen available.
Miniatures Rules
Firefight 2050 Rules
Patrick Todoroff offers Firefight 2050, a set of rules for 1/72 figures in a near future setting.
MIG Alley Ace
MIG Alley Ace are rules for modern aircraft combat from 1945 to the missile age.
Impetus Basic Rules Update
There’s been an update for the free basic rules for Impetus, a set of ancient and medieval wargames rules.
Flying Circus WWI Air Rules
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.