GHQ has a free pdf of its Beer and Pretzels Modern Micro Armor game, formerly a commercial effort, now a set of free wargames rules.
modern
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
Breaking News Free Modern Skirmish Rules
Breaking News is a set of free wargames rules for modern skirmishes. From the introduction:
BN recreates the tension of today’s battlefields, scenes of difficult operations that are often called “peace keeping” missions, with the additional presence of civilians and the media.
In BN the main elements are the men and their choices on the field – these rules are not a “brochure” containing all the features of modern weapons.
BN is designed for “one-off” simulations. Build up your unit with group of miniatures and spend other points on EXTRAS (support fire, intelligence, rules of engagement…)
Warzone Modern Battles
Warzone Modern battles is a set of free modern Wargaming rules for 1:72 or 20mm Squad/Platoon games.
PZ8 1950-69 6mm Wargames Miniatures Rules
PZ8 1950-69 is a set of free wargames rules for using 6mm miniatures to refight battles of the mid to late 20th century.