La Bataille d' Halle -1806 is a free print-and-play board game of the 1806 battle between French and Prussian forces near Halle.
Outlaws is a free print-and-play boardgame of gunfight skirmish combat in the Old West
Warlord Games is offering a free pdf of their Project Z zombie miniatures rules.
If you're a fan of John Carpenter's The Thing, you may be interested in this boardgame version of The Thing.
Here are a set of houserules for Osprey Publishing's Frostgrave.