Trireme, from Andy Watkins, is a set of free naval wargames rules. These are of the one-page variety, which means that there is much to recommend them. I also like the staggered movement system based on boat speed, which reminds me of the old Car Wars system.
Miniatures Rules
BloodBowl Hockey Variant
Here’s a variant for Games Workshop’s BloodBowl Game in a hockey format.
Battlestorm Miniatures Combat System
Battlestorm is a set of free wargames rules for medieval fantasy.
Free Black Powder, Hail Caesar and Pike & Shotte Rules
For a limited time, Warlord Games is offering free digital copies of Black Powder, Hail Caesar and Pike & Shotte
Tabletop Titans Super Hero Game
Tabletop Titans is a set of free wargames rules for superhero miniatures games.