Free, retro-ish, sort of OD&D-like, uses funny poly dice, lovely little pick-up-and-play RPG.
There's class based combat hit rolls, but to my surprise, there's no armour. D&D without armour?! Heresy!
Still it's a really nice pamphlet of rules -classes, spells, some monsters. Perfect for when away from home without any rules and maybe good to play with kids, or with drunk old schoolers.
Dungeon Raiders free PDF download on DTRPG:
(Art by Jaydot Sloane - http://jaydotsloane.tumblr.com/ )