Thunderbolt and Lightning is a set of free WWII air combat rules.
JD Burgess’ site has a list of major WWII air war campaigns. For each, he has lists of the planes that participated, with links to photos and information.
JD Burgess has a site with dozens of outstanding photos of World War II model airplanes. Each model not only has the traditional 3/4 on photo, but also left, right, rear, above rear, above left … well, you get the picture.
These would make a good painting guide for air war miniature wargamers.
Matt Unsworth offers a set of free wargames rules called The Few that he uses with his students to simulate the Battle of Britain.
Nigel Lancaster has written a set of abstract modern air war rules called Instant Thunder, and has graciously permitted MiniatureWargaming to host them. This is a major work, with 28 pages of rules, aircraft and missle lists, and designers notes.