The final version of Dark Dungeons, an OGL retro-clone of the best single book version of TSR's D&D game ever published, is now available in hardback and softcover from Lulu and as a free PDF. While the system rules are a fairly direct clone of the original RC, much of the campaign related material -- such as the structure of the planes -- is fairly original (due to copyright law).
Nevertheless, the author "Blacky the Blackball," has done a excellent job. There are rules for running fiefs and becoming an immortal for higher level characters making this the first retro-clone I know of which includes the 0e/1e/BEMCI "endgame." There are even rules for immortals that are much easier to understand and use than either of the Immortals sets published back in the 1980s and early 1990s. The new outer planes setup created for Dark Dungeons even allows for "spelljamming" campaigns similar to the 2e setting.
The PDF Version of the Dark Dungeons rulebook is 345 pages and is fully illustrated. Dark Dungeons is published under the OGL. Best of all, all the original text of the game is not only open game content but is placed in the public domain.
You can obtain a copy (hardcover, softcover, or free PDF) here: Get Dark Dungeons.
Dark Dungeons is not perfect, but with copies of the original RC getting more and more expensive, it's really great to see this. By the way, the name of the game is a deliberate play on the old anti-D&D Chick Tract from the early 1980s. The characters used in the examples in the rules are even named after some of the characters in the Tract. I love that touch.