The Napnuts site has a set of rules for conducting a Napoleonic Peninsular campaign. The Peninsular Campaign makes for a good campaign because of its relatively limited scope.
Maxims and Mausers is a set of free wargames rules for European Wars from the 1870s. From the website:
Maxims and Mausers covers European Wars at the moment. Inclusion of Colonial Conflicts is planned. These are very simple rules. That may be played either as a board game on a hex map or as a miniatures game on large hexagons decorated as terrain.
Mobsters is a set of roleplaying rules for 1920s gangsters. It’s a well-done and complete game.
The Dark Realm is a set of free miniatures rules for Dark Realm’s line of 6mm science fiction games.
Loyal reader Dentatus has contributed a set of free science fiction rules called Dead Run Redux.From the introduction:
Dead Run is a set of science fiction wargame rules. But not the science fiction of Star Wars or Flash Gordon; these worlds are hard, fast and deadly, and you need to be the same. Whether you’re facing down criminal scum, hordes of aliens, killer robots, twisted mutants, or waves of zombies, you just want to get the job done and live to tell about it. Sometimes you’ve got friends along for the ride – Runners they’re called – and sometimes it’s just you. Either way you’ve got to think on your feet and get the first shot off every time, because Runners don’t get too many second chances.