The Shade Isle Campaign
Labyrinth Lord, by Daniel Proctor, is a retro-clone which imitates the 1981 version of the classic D&D game (the so-called “magenta” Basic set by Tom Moldvay and the “cyan” Expert set by Zeb Cook and Steve Marsh). Labyrinth Lord deserves special mention (and its own wiki page here) over and above the other retro-clones, for a couple of reasons.
For one thing, it’s one of the more faithful clones out there. The differences between LL and B/X are all fairly minor, even if there are plenty of them (spell progressions, spell descriptions, experience tables, attack tables, and available armors are all tweaked: some to make them more like BECMI or AD&D or the d20 SRD; and others arbitrarily, so that LL is “just different enough” to be its own thing).
For another, it’s very popular and has a lot of support (not even counting Engines & Empires). Two books in particular are worth noting here: Original Edition Characters and the Advanced Edition Companion expand the range of games that Labyrinth Lord can emulate to include the 1974 original D&D rules and the 1st edition of AD&D. The AEC actually takes a lot of AD&D material and essentially converts it to be compatible with classic D&D, which is tremendously useful even if you’re not interested in playing an AD&D-style game.
Labyrinth Lord, Original Edition Characters, and the Advanced Edition Companion are all available for download from their publisher, Goblinoid Games. The Shade Isle Campaign only makes direct use of the AEC, from which a few magical items and spell effects have been drawn.