Games Workshop offers a free pdf of its Warhammer 40K battle primer, which contains basic for moving, shooting, fighting, using psychic powers and morale, along with other material to get you started.
Epic Escarmouches is an original wargames rules system based on Space Marine and Epic 40K.
Warhammer French and Indian War is a set of free wargames rules based on the Warhammer English Civil War rules.
Warhammer Colonials is a set of amendments to Warhammer 40K for 19th Century colonial warfare. from Jervis Johnson himself.
Warhammer Napoleonics is a set of stat lines and rules amendments for fighting battles of the Napoleonic Wars using the popular Warhammer rules.