Lion of the North is a set of free wargames rules for battles set in the Thirty Years War. The rules set also includes Haiduk and Rajtar for period battles in Eastern Europe, and a set of army lists.
Miniatures Rules
Maurice Lite Rules for the 18th Century
Maurice Lite is a sampler version of Sam Mustapha’s rules for warfare in the 18th Century.
Strange Aeons Eldrich Horror Skirmish Game
Set in the pulp 1920s, Strange Aeons is a skirmish game that sets bands of adventurers against Lovecraftian horrors. The game is commercial, but they have a free test version.
Roma Victa “Pulp” Rules For Ancients
If you scroll down on the Lead Adventure thread at the link, you will find a simple set of free wargames rules and a skirmish scenario for Late Roman “Pulp” Adventure.
Warrors of Fantasy Wargaming Rules
Warriors of Fantasy is a large scale set of fantasy rules designed to let you use any miniatures in your collection. The authors write:
�Warriors of Fantasy� is a Wargaming system that will be aimed at large scale battles using 100�s of miniatures per side. It will hopefully remain fast to play while retaining the depth of individuals. The �Warriors of Fantasy� system is geared towards representing medieval combat of all types, from realistic to high fantasy. The system is designed so you can use any figures from any manufacturer, these rules are written assuming you are using the 28mm scale (Games Workshop or equivalent). The system is designed to be rules-lite, with only about 10 or so pages of rules to learn to play.