Don offers an updated version of his air combat rules, which he now calls Aerial Craps.
Dogfight! is a set of free wargames rules from Don Gelwwe. The rules have a bit of a different flavor from most of the genre. Don writes:
These rules are an attempt to create for players a different experience in gaming WW1 aerial combat. They are meant to confront players with broader tactical decisions that reach beyond the immediate specific-maneuver-choices of a moment within a battle to deal with what is (or will be) happening within the scope of the engagement as a whole. The demands placed on players emphasize these larger decision-making aspects of a pilot’s energies rather than the common, traditional focus on flying of most other rules. By removing the need for the player to micro-manage the piloting of the aircraft (these rules assume that the little fellow in the model already knows how to do that…), gametime can be devoted to deciding what to do rather than how. Instead of choosing which specific maneuver to use to position one’s aircraft in a particular way, players will choose amongst tactical objectives (such as “attack that enemy” or “be over there”) and then decide on the risk-level undertaken to achieve that goal. The success of an effort (attack, defense, movement, observation, etc…) is determined by dice rolls that are influenced by the pilot / aircraft quality as well as the level of risk (which reflects the difficulty of the task attempted).
You also can discuss the rules in the forums here.
Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.
Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.
Matt Fritz offers a set of fast play miniatures rules for World War II Aerial Combat.