The folk at One Page Rules have released a new edition of GrimDark Future, a set of one-page rules (with lots of pages of armies you can also download)
Miniatures Rules
Battle of Salamis Rules
Matt Fritz offers a simple set of rules for recreating the 450 BC naval battle of Salamis between the Greeks and the Persians. The site even has links to paper models that you can use to play the game.
And All For One Three Musketeers Miniatures Rules
Eureka Miniatures offers And All For One, a set of free miniatures rules for playing swashbuckling miniatures games such as the Three Musketeers. The four page rules set is done in an interesting flow-chart style, making it very easy to follow.
Close Little World Wars Rules
Based in part on Donald Featherstone’s Close Wars, Close Little World Wars focuses on WWII Platoon level games.
Operation Sandtrap Plastic Army Men Rules
Operation Sandtrap is a set of free wargames rules for standard Plastic Army Men.