Lancer Runolfsson offers Six Page Napoleonics, a set of simple Napoleonics rules for a game in four to five hours.
rules
Ironclad Draughts One Page Ironclad Rules
Ironclad Draughts is a set of one-page rules for playing out battles between ironclads.
Air War 1917
Air War 1917 is a barebones — and free — version of Wessex Games’ AirWar: 1918 biplane combat rules.
Forge of War Science Fiction Rules
Forge of War is a set of free science fiction skirmish rules from Craig Cartmell. He writes that they were born of frustration with the Warhammer 40K rules from Games Workshop.
Elan Deluxe Napoleonic Rules
Elan Deluxe is a set of free rules for playing Napoleonic wargames. Once published professionally, it now is available online. This is a very well done, and interesting set of rules. The nominal scale is 1-50, with units representing battalions and regiments.