Meca Tac – The Giant Stompy Robots Edition is a set of free wargames rules for Mecha battles.
Miniatures Rules
King Of War Historical Ancient Battles
The Hour of Wolves and Shattered Shields site has a set of free wargames rules for Ancients Battles based on the free set of fantasy wargames rules called Kings of War.
Dungeon Tiles To Print and Play
Quests of Legend has a nice collection of dungeon tiles for you to print out and use for your role playing or miniatures games (or both!) in a dungeon crawl.
Planet 28 Narrative Science Fiction Rules
From Mammoth Games comes Planet 28, a set of free wargames rules (actually, pay what you want) for narrative science fiction games.
Mammoth writes:
Planet 28 is a simple set of sci fi skirmish rules for quick narrative games between warbands of warriors, mutants, and outcasts.
Inspired by the amazing work of kitbashers and converters the world over, planet 28 is intended as a pick up and play rule system for quick games when a longer rule system isn’t viable.
Rules include –
customer character creation.
basic weapons and custom weapon creation.
a story generator for setting up and adding narrative to your games.
a campaign system that lets you build and develop your warband between games.