The Wayback machine has stored a copy of the WRG 1925 to 1950 armor and infantry rules from 1973.
Micro Armor Devastation is a set of fast play miniatures rules for land and air combat from World War II onward. They’re very nicely done in a pdf format.
Assault Gun is a set of free wargames rules by David Kershaw. David writes:
This rule set is designed to allow players to simulate large actions of the Second World War using miniature models. Each division is represented by less than 10 bases (regiments and battalions), so players can command several divisions each, and play a recognisable action of the Second World War.
The rules originally were available on Yahoo Groups, but with that site shutting down in 2018, I have archived the files here.
Years ago, GDW used to hand out copies of a simple Frank Chadwick game called Battle for Moscow. I’ve got a copy, and have used it to introduce players to board style hex wargaming. If you dont’ have a copy of this classic, you can download it here.
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.