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.
Double Tap Spec-Ops One Page Rules
Double Tap is a set of free wargames rules for games involving special operations forces in the near future. The rules are just one page, but look as though they are anything but simplistic, as they use a reaction system.
Maxims and Mausers
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 Role Playing Game
Mobsters is a set of roleplaying rules for 1920s gangsters. It’s a well-done and complete game.
Deathball One Page Rules
Deathball is a set of one page rules for combat oriented football games. The authors write:
Deathball is a fast paced and brutal sports game inspired by others such as Dreadball and Blood Bowl. Challenge your friends in a match of this violent sports contest where a well-placed pass is just as important as a well-placed punch.