It occurred to me that the AGC Crew has mentioned several retro-clones on the show, but never really outlined which retro-clones are drawn from what material. So, I thought I would provide an outline of what's out there.
The below is taken, with some light editing, from RetroRoleplaying.com and Wikipedia:
What 's a retro-clone?
The OGL has allowed fans to recreate the rules of older, out-of-print editions of the world's most popular fantasy roleplaying game. These are usually called "retro-clones" because they are "clones" of the rules to older ("retro") editions of Dungeons and Dragons (or, as the authors will refer to it in the material for legal reasons, " the world's most popular fantasy roleplaying game"). While no retro-clone is an exact copy of an earlier game, they generally are close to identical in play.
Role-playing game publisher Matthew Finch was the initial author of OSRIC 1.0, which was afterward taken up by Stuart Marshall and released to the public in 2006 as a retro-clone of the first edition of Advanced Dungeons & Dragons (1977–1989). The release prompted another game designer, Daniel Proctor, to write and release Labyrinth Lord in 2007, a more complete retro-clone of Dungeons & Dragons B/X (1981–1982). The following year, Finch announced the release of Swords & Wizardry, ostensibly a simulacrum of the original Dungeons & Dragons game (1974–1977), and the retro-clone movement was pretty much launched.
Most retro-clones have free PDF versions and most have nicely bound printed copies available for a very reasonable price.
0e (Original Edition) RetroClones
EDIT: A listener pointed out that we neglected to list Lamentations of the Flame Princess, a 0e like RPG focused on "weird" role-playing.
MicroLite74 is Microlite20 (a trimmed-down, sub-miniature version of the Primary Fantasy SRD rules that has been designed to be quick and easy to play) that has been rewritten to recreate the style and feel of the 0e edition from 1974 (a.k.a the White Box). It takes the popular, very rules light 3.x Microlite20 and reshapes it to feel like 0e using Microlite20 rules as the base. There are two supplements, Ancient Auguries and Wary's Grimoire. Ancient Auguries adds an assortment of optional rules. Wary's Grimoire adds much of the material from the 0e supplements including more character classes and higher level spells.
A special note about MicroLite74: the author is offering a special MicroLite74 package as part of a fundraiser. He has medical bills exceeding $110,000USD due to cancer in the family, so if you're able to donate, please do so.
Swords & Wizardry is an OGL retro-clone of the 0e version from the mid-1970s, the same edition Microlite74 is loosely based on. Sword & Wizardry is handy for complex spell and monster descriptions for use with Microlite74 as well as a game in and of itself. There are two versions of this game. One uses just the material from the three original 0e booklets and the other uses selected material from the 0e supplements.
B/X, BECMI and Retro-Clones
The Basic Fantasy Role-Playing Game is loosely based on the so-called B/X edition of D&D from the early 1980s. There are a number of minor tweaks like ascending armour class and separation of races and classes.
Labyrinth Lord was written and edited by Daniel Proctor and published by Goblinoid Games. It's more closely based on the so-called B/X edition of D&D from the early 1980s (a.k.a the "Red Box"). Some minor changes have been made for compliance with the OGL and copyright law. However, there are a few differences between the two games. Rather than being released in separate Basic and Expert boxed sets like the version of D&D that it emulates, all of LL's rules are contained in a single volume. Another deviation from the source material is that characters can advance to 20th level (the 1981 Expert set only included levels up to 14). LL includes most of the same monsters, spells, and magic items as classic D&D, except for those designated as "product identity" by Wizards of the Coast. In some cases, these have been replaced with similar alternatives.
Dark Dungeons is based on the one volume "cyclopedia" version of BECMI edition. Like the "cyclopedia" version, this is a complete game in one volume allowing characters to advance through 36 levels and then try to become an immortal. Unlike most retro-clones, Dark Dungeons includes rules for high level characters with strongholds, mass combat, other planes, immortals (i.e. "deities"), and even includes rules for fantasy space travel.
B/X Companion is intended to be the third volume designed to complete the work begun with Tom Moldvay's Basic Rules and continued in the Dave Cook/Steve Marsh Expert Rules, both published in 1981. Both promised a "Companion" volume that would conclude the series. Instead TSR (and Frank Mentzer) re-wrote and re-published a five volume series (BECMI) that, while certainly interesting, failed to deliver on the promise of those earlier works. This is an attempt at producing the promised B/X Companion.
Companion Expansion is another attempt at producing the "third book" of the B/X version.
1e and Retro-Clones
Labyrinth Lord: Advanced Edition Companion is a supplement for the Labyrinth Lord game (see above). It is allows the use of the array of advanced character possibilities from the first edition rules, and first edition monsters and magical items. All character class options from the Labyrinth Lord core rules and the Advanced Edition Companion book are cross-compatible.
OSRIC (Old School Reference & Index Compilation) is an OGL version of the first "advanced" edition of D&D from the late 1970s and early 1980s. The first edition of OSRIC was designed to be just something that would allow publishers to legally publish OGL materials compatible with 1e. The second edition of OSRIC is much more complete and is written with both players and publishers in mind. More than a dozen distinct publishers have produced dozens of OSRIC-compatible adventure modules and supplements.
2e and Retro-Clones
For Gold and Glory contains all the information from the three core books of the 2nd edition of the world's most popular RPG, minus the optional rules. It's also organized in a much better fashion than the original.
Myth & Magic is another remake of the 1989 Second Edition Rules, and recently had a successful Kickstarter project. (NOTE: No free PDF version)
Double Zero is based on the 1983 Victory Games superspy RPG James Bond 007. It's developed by Berin "Uncle Bear" Kinsman.
Dragons at Dawn is a retro tribute to the very first fantasy gaming system pioneered by Dave Arneson. Dragons at Dawn is purportedly consistent with Arneson’s original, largely forgotten methods of play developed roughly in the period 1970-1973.
Four Colors System clones TSR's original 1985 Marvel Super Heroes game. This one's from Phil Reed of Ronin Arts.
GORE (Generic Old-school Role-playing Engine) clones Chaosium's Basic Role-Playing rules, the base system for RuneQuest, Call of Cthulhu, Stormbringer and several other classic-era RPGs. (For comparison, Chaosium offers free Quick-Start versions of Call of Cthulhu and its current edition of Basic Role-Playing, but these are free-as-in-beer, not open under the OGL.)
Legends of the Ancient World is a rules-light clone of Megagaming's The Fantasy Trip -- complete in seven pages. The game is a free PDF, but the publisher sells solitaire adventures for it similar to the ones Metagaming produced for TFT. Science fiction (Time and Space) and old west RPGs (Untamed West) based on these rules are also available as free downloads.
Mini Six is a fast and flexible rules system that encourages cinematic play. It's based on the D6 system from West End Games that was created in the 1980's for their edition of Star Wars. The free 8 page PDF includes OGL rules for fantasy, modern, and science fiction settings.
Mutant Future After Labyrinth Lord, Daniel Proctor wrote Mutant Future, which uses variant Laybrinth rules but captures the post-holocaust setting of TSR's Metamorphosis Alpha/Gamma World.
Spellcraft & Swordplay From veteran game designer Jason Vey, whose previous work includes All Flesh Must Be Eaten and Castle and Crusades, S&S is a re-imagining of Oe D&D with the original Chainmail combat rules based upon the 2d6 Man to Man table retained and expanded, and has been described as an interesting What If style of game
(NOTE: No free PDF version.) (EDIT: A listener has let us know that there is a free version of Spellcraft & Swordplay. You can download it here)
ZeFRS: David "Zeb" Cook's 1985 rules for the TSR Conan roleplaying game return in clone form as a generic sword-and-sorcery RPG complete in one volume. Trample the jeweled kingdoms of your own world beneath your sandals!