A list of freely-available, free game systems. Most are from members of the OSR, and available under the OGL. Everyone has their own interpretations (Ascending AC? THAC0? Should there be x class?), but everything’s pretty much modular anyway. Take what you want, leave the rest, and run it with a beer in ye.
Cloned systems are arranged by the publishing date of the original game they are based on.
Oh, and the language here is arcane, yes, but we’re all covering our rumps here. “The Game” refers to the one we all think of when you say “RPG”, that is owned by a large and litigious corporation who is currently saying “Happy Birthday” to the ground.
Labyrinth Lord Equivalent to the adjectiveless 1981 edition. “Advanced Edition Companion” is equivalent to the first edition of the advanced version of “the game”.
Blueholme – Blue. Holmes. Need I say any more?
Mazes and Minotaurs A “what-if” game – what if Classical mythology (not Medieval European) informed The Game?
Vikings and Valkyries A Norse world based on M&M
Original Edition Delta Properly a set of house rules, but still very extensive. The gent in question also has a nice set of mass-combat rules and some other toys available.
OSRIC Recreates the same edition as the AEC for Lab Lord.
Lamentations of the Flame Princess: Grindhouse Very loosely related to Basic. Jim has taken the palette given and violently hammered it into something.. well, even more fun. And dark, and probably tragic.
Mutant Future – Post-apoc fantasy, from a world with an affinity for short wavelengths.
Talislanta Not an OGL game: the creator of Talislanta put it all up on the web. Never played, but the generosity is much appreciated.
Open D6 The system used for the original West End Games Star Wars system, one of my all-time favorites: I used a home-ruled D6 as my go-to universal system from 1990 to about 2001 (when I found Big Eyes, Small Mouth). The game was recently released under the Open Gaming License, and the linked page contains all the SRDs. Also available on RPG.net, for free, at slightly better quality.
There’s also MiniSix – brought to my attention by a commenter – which is, well, basically Perfect20 for the WEGd6 system
Interlock Unlimited The core system for Cyberpunk 2020, Mekton (as well as II and Zeta), Bubblegum Crisis, and much more besides. Not an OGL system, but the creator actively encourages fan-made content.
Tri-Stat DX A free system developed from Big Eyes, Small Mouth. Not released under the OGL. Plays a little differently, but you can still run a hell of a game off her.
D20 SRD Lets you play edition #3.5, as well as hundreds of games that are utter carp and a tiny handful of decent games.
BFRPG A rules-lighter version of 3.5. Not to be confused with Chaosium’s BRP system.
Modified “anime” SRD Used for BESM D20, among others. Guardians bit the dust, alas.