While we're talkin' old school, I would be remiss if I did not mention the very exciting release yesterday of Swords & Wizardry by Mythmere Games, in essence a "0e" D&D with a couple of tweaks here and there. The pdf is a free download, and the print version only $7.50. I'll let this quote from the Swords & Wizardry site give you an idea of part of the philosophy behind this project:
"Start with the simple framework of these first rules, and then imagine the hell out of it".
For more, just read the mission statement. I may not agree with everything in there, but I hardly have to. Rock on, and best of luck!
And I want to say as far as old school goes in general: Swords & Wizardry, Microlite '74, Castles & Crusades, Basic Fantasy, Labyrinth Lord, OSRIC, Mazes & Minotaurs, Hackmaster, Mutant Future, or ant of the original boxed sets themselves--hey, if you're having a blast with it, then you're aces in my book. You will never see me denigrate anyone on this site for their choice of old school goodness. That's not what we're about here. If I may, I'd like to just reiterate perhaps the most important life lesson I've had: