Dave Ferris offers Panzer III, a set of free wargames rules for WIII tank battles.
WWII
Firefly Outgunned WWI Microarmor Rules
Firefly Outgunned is a set of free wargames rules for WWII microarmor. They’re based on the old Firefly and Challenger rules sets.
Naval War World War II Rules
Naval War is a set of free wargames rules for World War II naval actions.
Combat Captain WWII Rules
Combat Captain is a set of free wargames rules in which the basic unit is a company consisting of up to eight squads. Author David Raybin says that the rules “allow a high degree of flexibility in unit activity during a turn. The idea is simplicity of play so the game will move along to a satisfactory conclusion in under three hours.”
The Devil’s Brigade WWII Rules
The Devil’s Brigade is a set of free wargames rules for WWII skirmishes.