Before getting to the links, let's have the ground rules.
First, since there's no definition of what is or isn't old-school, I'm going to be expansive. The line between retroclone, simulacrum, and old school-evocative is blurry. I prefer to be inclusive rather than exclusive. I've indicated which edition of D&D a game emulates, to the best of my ability. Some of them aim for a generic old-school feel rather than to simulate a specific edition.
Second, I give my impressions where possible, but I haven't played all of these games or even read all of them cover to cover. I'm working my way through them one at a time. Where I have no words of my own to say, I've used the authors' and publishers' words to describe the items.
Third, these are the materials that I've collected. I'm sure many items that belong here aren't listed. Forward the links to me and I'll add them if they fit in.
Currently my focus is on free material, especially RPGs. Numerous categories are missing, such as online, old-school dungeon generators. I'll get to them before too long. At some point I will expand the section for commercial rules, but not until I have the time to tackle it correctly. I don't want anyone dropping (or not dropping) ca$h on a product based on a half-baked comment here. Cash products deserve thoughtful reviews. For now, this is just a capsule-comments-and-links 'gator.
Free Swords & Sorcery RPGs
Bandits & Basilisks: This isn't an RPG; not really. It's more like six pages of ideas that might spur some variants for whatever old-school RPG you're currently playing.
Barbarians of Lemuria: Not a clone of any particular game. Barbarians of Lemuria uses a simple 2d6 system to emulate an old-school feel with the benefit of some modern design ideas to smooth things out. I'm not 100% certain, but I believe this free PDF is an earlier, shorter, less polished version of the "Legendary Edition" that's offered for sale.
Basic Fantasy: "The Basic Fantasy Role-Playing Game is a rules-light game system based loosely on the d20 SRD v3.5, heavily rewritten with inspiration from early RPG game systems. BF RPG is an Open Source game system, supported by dedicated fans worldwide."
Blueholme Prentice Rules: (emulates 1977 Holmes-edition D&D) "A tabletop fantasy roleplaying game that emulates the game play of the original basic rule book, popularly known as the Holmes Edition or simply the Blue Book. The rules in this book allow for characters of 1st to 3rd levels and include everything the referee could possibly need to create and run a campaign in the Underworld: monsters, magic, treasure, and … well, what more do you need?"
Dangers & Dweomers: A Swords & Wizardry variant, more fleshed out than the standard version of S&W.
Dark Dungeons: (emulates the D&D Rules Cyclopedia) Many people revere the Rules Cyclopedia as the acme of all RPG products, ever. Like many holy grails, copies of the original are now hard to come by. For those who can't afford an original Rules Cyclopedia, Dark Dungeons is an excellent stand-in. DD is a good alternative to Labyrinth Lord or Adventurer, Conquerer, King if you're looking for a BECMI-style game at high levels (16+) with strongholds, war, and eventual immortality for PCs. Available as a free PDF in standard or print-friendly versions, or as print-on-demand in softcover, hardcover, and deluxe hardcover versions.
Dungeonslayers: "The rules of Dungeonslayers were designed to be very basic and simple, to bring the charming flair of old-school gaming back to life. Dungeonslayers is about straight-forward plots in your traditional fantasy world, where evil is still evil, where monsters have to be killed mercilessly, where devious traps strike and where phat loot awaits, while pencil and graph paper work their own special magic around the gaming table."
Epees et Sorcellerie: (inspired by OD&D) The original version of E&S was published in French. It took a couple of years before someone translated it into English, and we're all richer for the effort. At first glance, it's just another OD&D-inspired variant game, but take a longer look. It's remarkably well done and loaded with old-world charm.
Flying Swordsmen: This is something different. It's a D&D retroclone, but the setting is fantasy China and the emphasis is on over-the-top martial arts stunts. I haven't had a chance to delve into it very deeply, but it looks like it could be loads of fun. Well worth a download.
Holmes77: (emulates 1977 Holmes D&D) A retro-re-edit/re-org of Holmes's re-edit/re-org of OD&D. "The ... document includes rules and guidelines as laid out in the 1977 edition of the revised game, Dungeons & Dragons. Since this information was directly transferred and/or interpreted from the original set of rules appearing in 1974-75, what follows is an attempt to clarify those things that are implied within the '77 document and to provide additional data to support the implications." A handy reference to have around. (This link no longer works, and I can't find the game elsewhere on the web. Does anyone know whether these rules are still available? Are they related to the BlueHolme Prentice Rules?)
Labyrinth Lord: (emulates 1981 B/X) Labyrinth Lord is one of the OSR leaders. It pays homage to the 1981 B/X rules with slight modifications and clean-ups. I'm a huge fan of 1981 B/X, so this is one of my favorites among the retroclones. You can get a free PDF with blank spots where the illustrations belong (good for making notes), or pay a bit for the full version. Also supported with an excellent magazine (Oubliette).
Lamentations of the Flame Princess: (emulates 1970s/early 1980s D&D in a general way) LotFP is an excellent but potentially infuriating game. It plays extremely well and incorporates some clever innovations to old-school-type rules -- I can't stress that enough -- but your opinion of the rulebook will be colored by your reaction to the author's voice, which is ever-present. Grab this free, no-art version and see for yourself what I'm talking about. Also available in a boxed version that is quite nice, except the print is painfully tiny to my aging eyes. One advantage to the PDF is that you can zoom in as much as you need to on a screen. Recommended.
Mazes & Minotaurs: (emulates OD&D and AD&D in different editions) Overlook for a moment the fact that I geek all over games themed around the heroic age of Greece. Even without that to hook me, M&M is an amazing body of work. It's a solid, flavorful, playable game; it's a hilarious parody; and it has more -- and better -- supporting titles than a lot of professional, for-pay RPGs. I would need an Excel spreadsheet to fully show you just how much you need to download Mazes & Minotaurs this very moment.
Microlite74: (emulates all early editions of D&D in different versions) "The goal of Microlite74 games is to recreate the style and feel of that very first ("0e") fantasy roleplaying game published back in 1974 without giving up all of the clearer mechanics of modern D20-based versions. The rules are not intended to be a clone of the 0e rules, but rather a conversion of them to a rules-lite D20-based system that encourages old-school play without strictly old-school rules." Three versions of the rules are available -- basic, standard and extended -- plus more companions and expansions than you can shake a stick at. ML74 is RPG crowd-sourcing at its best.
OSRIC: (emulates AD&D) Where most retroclones and old-school emulators try to evoke OD&D, Holmes Basic, or 1981 B/X, "The Old School Reference and Index Compilation" aims at AD&D. The PDF is big, as you might expect from a document that entails the PHB, DMG, and MM. But it's one of the cornerstones of the OSR. It's not, however, just a set of rules. True to the OGL, anyone can offer supplementary material for OSRIC, so an impressive body of support products now orbits around this sun.
Peril: A minimalist game accompanied by minimalist adventures, all of which forms a very cool little package. I can't say whether Peril is in its final form yet; it seems to be a work in progress by a perpetual tinkerer.
Searchers of the Unknown: (emulates OD&D) With just one page of rules, SotU is easily the minimalest of the minimalists. And it's a terrific game. You could play this one-page game for a long time before it would get old. If it does get old, there are plenty of one-page expansions to keep it fresh.
Spellcraft & Swordplay: (emulates OD&D) What sets Spellcraft & Swordplay apart from other OD&D clones is that it uses the Man-to-Man Combat Table as the basis for combat instead of the more widely accepted Alternative Combat System that has become the standard. The basic version can be had for free.
Sword & Shield RPG: Minimalist rules, just six pages long (as of this writing), plus a handful of variants. By the same author as Peril.
Swords & Wizardry: (emulates OD&D) If you have just one retroclone on your ereader or shelf, it should be Swords & Wizardry. S&W begins with the ideas of OD&D and ends with a clean, organized game that plays like buttah. IMO, Mythmere embodies the very best aspects of the OSR. Ruins & Ronin is a (standalone) oriental variant for S&W. It costs $1, so technically it's not free, but come on. I'm including it here.
Torch and Sword: This game appeared in July 2011 in beta form and hasn't progressed beyond that. Even as-is, it's solid and playable, and it crams a lot of minimalist ideas into 79 pages.
Under the Moons of Zoon: Another game from Luigi Castellani, the mad Italian who also put together Dangers & Dweomers. Under the Moons of Zoon is a must-have, IMO. Bear in mind that the only genre I like better than Conan/Thongor/swords & sandals pulp and the Heroic Age of Greece is planetary/lost world romance. UMZ scores on that goal. This game is a delight. Visit his site and you'll see that Castellani is a busy man.
Warrior, Rogue & Mage: Those aren't the names of classes; they're your character's attributes in WR&M. This is a quirky but well done game from Stargazer Games. Undoubtedly it stretches some people's concept of old-school all out of whack. WR&M incorporates more modern indie game design than old-school thinking. But the feeling of the whole is very old-school. Beyond that, the art and production in this free download blows away many for-pay PDFs. While you're at the site, check out the top links to other Stargazer games. (One of them is a gunpowder expansion for Dungeonslayers.) Most of the other offerings are very modern in design so they don't fall under the OSR banner, which is why I'm not going into them individually here. They're all worth a look, however. I find A Wanderer's Romance especially intriguing.
Not-Free Swords & Sorcery Games(About which I hope have more to say eventually.)
D&D Basic Rules and D&D Expert Rules, 1981 edition (It doesn't get much more authentically old-school than actual D&D)
Adventurer Conqueror King
Barbarians of Lemuria
Crypts & Things
Dragons at Dawn
Dungeon Crawl Classics RPG
Lamentations of the Flame Princess
The Secret Fire
Free Science Fiction RPGs
Star Frontiers: This site offers very, very close clones of the original Star Frontiers rulebooks, adventures, magazine articles, even the Star Frontiers Endless Quest books. It took me three decades to make peace with Star Frontiers (long story; I'll tell it here sometime), but now that I have, I've become a big fan. Once you have the game -- or even if you don't but you just like SF RPGs -- check out the excellent Star Frontiersman Magazine, from the same people who publish Barebones Fantasy (you'll note some similarities in the foundational rules between SF and BBF). Star Frontiersman was instrumental in getting me re-involved with Star Frontiers, and for that I thank them heartily.
Encounter Critical: Encounter Critical is written as if a certain small group of innovative game designers in the 1970s had chosen to write an SF game instead of a fantasy game. You don't need to be in on the joke to enjoy EC, but it helps.
Mutant Future: From Goblinoid Games, the same outfit that produces Labyrinth Lord, and its heritage shows in quality. MF uses the same underlying rules as Labyrinth Lord for post-apocalyptic science fantasy. You know which game that is.
Stars Without Number: Not a retroclone but a new interpretation of what an old-school SF RPG should be like. Deserving winner of the Three Castles Award for 2012.
Terminal Space: "The aim of Terminal Space, supplement for the Original Edition roleplaying game, is to add a new ruleset to help transfer the game to an entire new dimension which space exploration is. Any additional materials were created with a thought on settings heavily based in Science Fiction and Science Fantasy genres than most of the materials created for the game so far."