Thunderbolt and Lightning is a set of free WWII air combat rules.
Rules: WWII
Blitz Miniatures Rules For London Firefighting 1940 – 1941
From Jim Wallman comes Blitz, one of the more imaginative miniatures rules sets I have encountered. In it, you command squads of firefighters coping with the dangers of the London Blitz of 1940 – 1941.
Science Fiction Company Action Rules
The Grid Based Wargaming Blog has a set of free wargames rules for company level units using 28mm models and a 6 inch grid.
WRG 1925 – 1950 Armor and Infantry Rules
The Wayback machine has stored a copy of the WRG 1925 to 1950 armor and infantry rules from 1973.
Wierd World War II Rules For Two Hour Wargames’ Nuts!
Wierd War Tales is a free expansion for Two Hour Wargames’ Nuts system. Covering all of the previously unknown aspects fo teh Seocnd World War, including the involvement of rocket troops, zombies and vampires, it looks like a lot of fun.