At this point, it seems as though Mantic has stopped supporting the Mars Attacks rules. If you’re interested, though, you can get the playtest rules at the link.
Zombie Assault is a board-figure-and dice-game in which the players s search the board for equipment which helps them fight their way to the saving car or to barricade in the house. In the meantime, Zombies try to eat their brains.
Seven Years Warmaster is an adaptation of Games Workshop’s Warmaster rules for the Seven Years War.
A Spot of Friendly Wizardry is a set of free wargames rules for battles between two or more wizards.
Grunt, Growl and Tear is Invisible City’s free board game of monster combat. It’s “A no-holds barred game of monster combat where you can rip limbs from your opponents and add them to your body, making yourself more powerful.”