Mustangs is a classic Air Combat game from Avalon Hill which, like most of their great systems is long out-of-print. Warflag, however, has a set of rules for playing the game in miniature.
WWII
Naval Action
Naval Action, by Lou Coatney, is a set of free wargames rules for miniatures battles in the World War II Pacific Theatre. The page also provides some markers to use in place of model ships.
Juggernauts: Panzerkeil vs. ArtKor, Kursk 1943
Juggernauts: Panzerkeil vs. ArtKor, Kursk 1943 is a print-and-play, hex-and-counter wargame
Death Struggle: Barbarossa 1941
Death Struggle: Barbarossa 1941 is a print-and-play, hex-and-counter board game from prolific game creator Lou Coatney.
High Seas Drifter WWII Era Battleship Rules
High Seas Drifter is a set of free wargames rules for playing battleship combats in the World War II era.