MIG Alley Ace are rules for modern aircraft combat from 1945 to the missile age.
There’s been an update for the free basic rules for Impetus, a set of ancient and medieval wargames rules.
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.
Banners and Billhooks is a set of free wargames rules for historical “imagi-nation” wargaming in the medieval period.
Arthur Chang has written a set of add-on rules for playing massed battles with the Wizards of the Coast rules for Star Wars miniatures.