Swords & Wizardry yet. You'll probably know it, but if not, it's worth checking out. Especially if you like Old School gaming, or D&D in its first incarnations. Or, if you have no idea what I'm talking about and are looking for a good and simple role playing game.
You might also just download Dark Dungeon 2nd Edition here. But if you want more options, Swords & Wizardry is a good place to start looking. It's written by Matthew Finch, who also wrote the liberating Quick Primer to Old School Gaming. Personally, it gave me most of the old D&D feel of all retro-clones I've looked at so far. Even if Labyrinth Lord is a bit more streamlined, maybe.
If you prefer even closer to 0e, the very first edition of D&D, then you might like S&W White Box. Which is real good too. You'll just have to decide more on which optional rule you want to use. So you might not want it as your very first role playing game.