GHQ has a pdf of its Beer and Pretzels WWII Micro Armor, originally a commerial set, now a set of free wargames rules.
WWII
OP Art – Operational Rules For WWII Eastern Front
Op Art is a set of rules for operational level games on the Eastern Front of WWII. The author writes:
Operational Art is the term first coined by the Russians to describe the level oc fomat that falls between the tactics used on the battlefield and the strategic direction of a war. The basic units of manoeuvre are division sized formations and the decision making level is that of army commander.
The rules have been designed so that players can recreate battles fought at this level; as a result a degree of abstraction has been included in the mechanisms used and especially in the way in which combat is resolved.
Panzer Gruppe WWII Rules
Panzer Gruppe is a set of free wargames rules for corps/army sized battles in World War II.
RKAA Brigade Commander Rules
RKAA Brigade Commander is a set of free wargames rules for brigade sized encounters in World War II on the Eastern Front.
Pz8 Wargames Rules
This 24 page pdf has Pz8’s relatively simple free wargames rules for a variety of periods, including
Colonial
WW1 Divisional
WW2
WW2 Divisional
1939 – 1941 Naval
WW2 Coastal
1935 – 1965 Aerial
1965 – 1075 Aerial
1950 – 1975
1975 – 2010
Science Fiction
Space Combat