Possibly this one needs no introduction to you, as this one won this year's ENnie award. I'll chance that.
Old School Hack is an easy going, beer and pretzel style take on what 1981 D&D might have been. What it might have been if the designers would have hated intricate dungeon mapping, three digit math, and rulebooks heavy enough to knock an Umber Hulk out. Instead they would have loved sweets, easy rules, fast paced combat, a cheery atmosphere, comic style play, and a cooperative group spirit.
One of the nicest touches of the system I think, is the awarding of awesome points. These are a sort of luck points, or fate points, as many systems know them. But in this case players may also award them to eachother, for cool play moments. Also, the game master can deal out more nastiness, while giving more awesome points to compensate. Awardable awesome points are kept in a bowl, possibly as sweets or other edibles. To me it's a refreshing system mechanic to find in a dungeon delving game.
Old School Hack beta 1.0 is available for free, and worth to give a spin in your group. Be sure not to eat your awesome points before you use them though. If you want, you can read some interviews with writer Kirin Robinson here and here - or have a look at the Red Box Hack inspiration for this game by Eric Provost. And despite his busy private life, Kirin intends to expand on the game soon. Which seems an excellent idea to me.