Matt Fritz offers a free set of miniatures rules for playing games involving B17 bomber runs over Germany in World War II. Looks like fun.
Phantoms is advertised as a quick and easy air combat game covering the period 1960 to the present.
Typhoon is a set of free wargames rules for modern jet combat. From the author:
Typhoon is an introductory set of modern jet combat rules that does not require hex mats or any special flight stands for miniatures. It is designed as a quick playing, low complexity game for gamers who are interested in flying multiple aircraft with aircraft from the late 70s up to the Eurofighter and F-35.
Spandau and Lewis is a set of free wargames rules for WWI aerial dogfighting.
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.