Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.
Luft ’46 is a site that offers a wealth of information on aircraft projects that the Germans had in line is World War II had lasted a year or more longer. Fascinating stuff. I’d like to see some of these show up on a wargames table.
MIG Alley Ace are rules for modern aircraft combat from 1945 to the missile age.
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.