The downloadable D20 system reference documents provide most of what you need to play any of the d20 system games that stem from the original Dungeons and Dragons system.
Microlite 20 is a set of minimalist RPG rules designed for compatibility with Old School Dungeons and Dragons materials, as well as the d20 OGL. Fourteen free Microlite 20 variants are available at the link.
Here is a collection of OGL MicroLite 20 RPGs. From the website:
The new two volume 2012 Edition of The Microlite20 RPG Collection is now available for free download. The Microlite20 RPG Collection is a PDF compilation of Microlite20 based games. This collection is made available for free and is not to be sold. Each individual game is copyrighted separately, is based on the Open Game License (because the original, Microlite20, is OGL), and is available for free. Each game is a separate entity with its own copyright and license. The author of each individual game is solely responsible for his or his work. No editing of individual games has been done. The original 2010 edition was a single volume with around 600 pages. The 2012 edition is two volumes with a total of over 1300 pages.
In addition to the standard Microlite20 rules (and a large number of optional systems and expanded rules, The Microlite20 RPG Collection (2012 Edition) includes the following Microlite20-based role-playing games: Microlite20 House Rules, MicroFantasy, Alter Microlite20, Ultramicrolite20 Revised, Ultramicrolite20 Revised II, Nanolite20, Microlite20 Modern, M20 Modern: Expert, Microlite20 Modern-Day, M20 Hard Core Rules, Swords against Sorcery, M20 Heroic, Iron Heartbreakers, Microlite11, WildWalker’s M20 4e, Microlite20 Variant 4e, Microlite 4E, Micro Action Fantasy, MULRAH, Lite20, Mini20, Realms of Renown, Microlite74: Basic, Microlite74: Standard, Microlite74: Extended, Microlite74: Companion I, Microlite77, Microlite20 OSS, Advanced Microlite20 OSS, Argo, M20 Hyborian Age, Microlite Conan, Prehistoric Microlite20, Microlite20 Bronze Age, Microlite Dark Sun, Omerian Tales, Beacon, WarEngine RPG, Yamato M20, Challenges & Champions, Microlite20 Elf Lords, Microlite20 Resident Evil, Microlite20 Cthulthu, Microlite20 Vampires, Microlite20 2012, SpyLite, Giant Bug Invasion, Cyberpunk, SuperLite, Microlite20 Costumes, Tumbleweed, Gunsmoke & Goblins, Owl Hoot Trail, Microlite Storytelling, TileHack, Dragoons, ZombiePocalypse, Relics & Ruins, OmegaLite20, RABID, Microlite20 Vermin, Where No Man Has Gone Before, Galactic Methuselah, FrontierLite, M20 Star Wars, Microlite20 Star Wars, Scions of a Primordial Planet, Micro MechWarrior, Stargate 1895, Blaster D20 Microlite, Pendragon D20 Microlite, Diabolical D20 Microlite, PathfinderLite20, Microlite20 Golden Edition, DungeonFinder Player’s Guide, DungeonFinder GM’s Guide, DungeonFinder Book of Monsters, and Grimm Lite.
Under the Moons of Zoon is a John Carter of Mars inspired free RPG.
The author writes:
Under the Moons of Zoon (also UTMOZ) is the grand child of a previous project of mine Kalevala.
Kalevala was originally born in 1991 as a one of many cheap italian role playing, live action and board games collectively known as “Giochi del duemila”. Of all these mini-games Kalevala was chosen to be published in Kaos a games’ magazine published at the time by Nexus Games.
Kalevala used a sort of universal mechanic based on 1d20 and used a free-form magic system simialr to that of early versions of Ars magica.
Later on -circa year 2000- Kalevala got translated to English and completely revamped to use only d6s for publishing under Demian Katz’s Aegis.
While rewriting kalevala I had the distinct impression that the new system had potential, and already at that time I had a compulsive need to write COMPLETE rulesets, as complete as my beloved D&D Rules Cyclopedia.
UTMOZ was born from the revamped system for Kalevala, adding psionics, weird science and much more and removing magic. The main inspiration for the background setting is John carter of Mars by Edgar Rice Burroughs.
Edgar Rice Burroughs’ Barsoom novels are among my favorite guilty pleasures. Sean K. Reynolds has a page with material, including monster and character profiles from his d20 Barsoon campaign.