After all, sometimes, you just want to play the original games without modern hacks. Or you want to hack it yourself.
This is likely not a perfect list of faithful retro-clones; only the ones I have personally had a chance to read and make some notes on.
Almost all of these are re-written, reorganized, and in order to avoid copyright infringement, have deliberately altered some numerical values or table structures.
Swords & Wizardry Complete is a recreation of the 1974-1978 rules and expansions that is pretty faithful, although it tends to pick and choose between alternative systems to create a single, unified game, and imports initiative systems from later game editions. It also includes some optional modernization rules, such as an ascending AC and Challenge system. My review is here. The corebook is available free from Frog God Games or for $1 on Drtivethru RPG. Hardcopies can be purchased on Amazon.
Whitebox Fantastic Medieval Adventure Game
Smoldering Wizard. Hardcopies are sold at cost on Amazon or in pdf or premium hard copy on DrivethruRPG
Basic D&D Clones
Blueholme, a.k.a. Blueholme Prenice Rules from Dreamscape Design is a reproduction of the original 1978 "Holmes" version of Basic Dungeons & Dragons using a cleaner presentation. The expanded Blueholme Journeymanne Rules includes character advancement to level 20 and includes its own take on wilderness survival and dominion rules, along with level-appropriate monsters and treasures. The Journeymanne rules use the later Moldvay Basic Dungeons & Dragons as a source, but tries its best to keep to Holmes' vision, and incorporates a lot of Holmes content. Blueholme is sold through Lulu and DrivethruRPG
Labyrinth Lord from Goblinoid Games includes the Modvay/Cook era (1981) version of Basic and Expert Dungeons & Dragons. An art free version is available free online, while a beautifully illustrated version can be bought in pdf or pod through DrivethruRPG.
Goblinoid also offers an "Advanced Labyrinth Lord" that adapts and resolves many AD&D rules to be used with a Basic framework. Goblinoid also a clone of TSR's D&D based Post-apocalyptic Gamma World.
Old School Essentials from Necrotic Gnome, This is another Moldvay Basic/Expert clone that includes some optional rules for modernization, such as Ascending AC. OSE is incredibly popular due to Gavin Norman's flare for excellent presentation and organization, making this the easiest B/X clone to learn and reference on the market. OSE has a downloadable basic ruleset for free and an online SRD, but otherwise must be ordered online through Exalted Funeral for hardcopy or DrivethruRPG for digital.
The Old School Essentials Advanced Fantasy Adventure Game rules sourcebooks, much like Advanced Labyrinth Lord has guidelines for incorporating some of the innovations of AD&D into a B/X framework, including reimagined AD&D classes, both separate race-as-class options, and additional racial classes. This is split across a player's and a referee's rules tome.
Dark Dungeons by Gurbintroll Games is a complete compendium of the Mentzer/Heard/Allston era of BECMI Dungeons & Dragons from 1983-1991. So it includes everything you will find in the Rules Cyclopedia and some abbreviated rules. This game is free as a PDF on DrivethruRPG. A newer version entitled Darker Dungeons was later released by Gurbintroll that simplified and unified BECMI into a less-faithful but simpler game, which was later combined with Dark Dungeons into a new Dark Dungeons X.
Advanced D&D Clones
One of the first retroclones, Old School Reference and Index Compilation is an accurate compilation of the three core rulebooks of Advanced Dungeons & Dragons' mechanics, although it does not include much of Gygax's non-mechanical campaigning advice. OSRIC is free to download, and can be purchased in softcover from Lulu, or hardcover brim Black Blade.
For Gold and Glory from God Emperor Games is a very faithful reproduction of Advanced Dungeons & Dragons 2nd Edition, This is a fairly striaghtforward reproduction of the content of the original AD&D2e core manuals, although it excluded some of the optional rules from AD&D2e for brevity. Its own sourcebooks add new twists and options. For Gold and Glory is free to download on DrivethruRPG. And may be printed POD as well.
Pathfinder (1st edition) is a reproduction of Dungeons & Dragons 3.5 Edition, with a few simplifications, for example taking the formulae for grappling, tripping, disarming, etc., and abbreviating them into fixed Combat Maneuver bonuses and defense numbers. There are only a couple of minor changes to actual mechanics, many of them streamlining feat trees and tweaks to how 0-level spells work. Pathfinder excludes prestige classes. It is often referred to as "D&D 3.75e" for these minor modifications. Pathfinder has a huge following, as there was a massive migration of D&D fans to it when Dungeons & Dragons 4th Edition was released.
The newer Pathfinder 2e has changed substantially from Pathfinder, and is not fully backwards-compatible. It might be considered a Parallel evolutionary branch of Dungeons & Dragons.
Pathfinder is sold in most brick-and-mortar book and hobby stores and can be readily ordered online through marketplaces like Amazon. All rules are available for free, even from expansions, by using the Pathfinder SRD.