Flying Circus is a set of free wargames rules based on the old Avalon Hill Mustangs game.
Rules: Aircraft
Angels 1-5 World War II Air Combat Game
Angels 1-5 is a game of air combat over Britain in 1940
Zeppelins Over London
The endlessly interesting Junior General site has a new game called Zeppelins Over London.
Thunderbolt and Lightning WWII Air Rules
Thunderbolt and Lightning is a set of free WWII air combat rules.
The Few: Battle of britain Fast And Easy Rules For Students
Matt Unsworth offers a set of free wargames rules called The Few that he uses with his students to simulate the Battle of Britain.