The Grid Based Wargaming Blog has a set of free wargames rules for company level units using 28mm models and a 6 inch grid.
The Wayback machine has stored a copy of the WRG 1925 to 1950 armor and infantry rules from 1973.
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.
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.