Since lots of Pathfinder players are checking out this blog, I realized that many readers may not know what I'm talking about when I mention the "Swords & Wizardry" version of Rappan Athuk.
Swords & Wizardry is a retro-clone of 0E. That's original D&D, the version(s) of D&D that actually preceded what's usually referred to as First Edition, or "1e." A retro-clone is a rewritten version of the underlying rules of the earlier game*. If you want to play the original version of the game, Swords & Wizardry is a very, very close facsimile -- there are some minor distinctions where there was a content-gap in terms of what could legally be included, but even these are pretty close.
The Core Rules of Swords & Wizardry are available as a free download, and so is the "WhiteBox" version which covers only the rules of the boxed set (none of the supplements). There is also a not-free but inexpensive "Complete" Rulebook that's published by Frog God Games and which is based on all the 0e supplements. I realize that three versions of a game is terrible brand-management, but I'm a much better gaming-geek than brand-manager, so that's how it is.
Quite a few small publishers sell modules for Swords & Wizardry, although the two big sources of material are Frog God Games and Mythmere Games. The game has a weird and tangled history, but that's perhaps a tale for another time.
In any case, when I talk about the Swords & Wizardry version of Rappan Athuk, that's what I mean: the Original 0e rules of the game, circa 1974-1978 depending on which version of it you use.
*Retro-clones generally use the Open Game License to access copyrighted material that is also OGC, and additionally draws upon the specific rules of the earlier game that aren't subject to copyright. There's a LOT more to it than that in terms of the legalities, so don't try this at home without learning a lot more: every big retro-clone I know of has brought lawyers into the design process to make sure it's being done properly.
Quick note: Whenever I mention any comparison between Swords & Wizardry and "D&D," I should mention that S&W is obviously NOT compatible with the currently trademarked "D&D." Don't take anything I say to mean that there's trademark compatibility under the terms of the Open Game License -- Swords & Wizardry is NOT compatible with the existing WotC trademarked game.
First Design Note
Second Design Note
Third Design Note
Fourth Design Note