The Nine and Thirty Kingdoms: Liber Zero

Liber Zero is theoretically a retroclone of old school class-and-level exploration fantasy adventure games. It is compatible with several early games and retroclones, primarily Gygax and Arneson's original RPG, but it's designed with the following features in mind:

Distilled -- Patterns in the old-school games become broader principles for elements like classes, races, monsters, and magic.
Generic -- Terms were shortened or replaced in some cases if they were too specific to one old school game.
Simplified -- Irregularities in things like class advancement or combat were removed.
Fiction-Focused -- Default assumption is that players do not need to learn any rules. They play the game by imagining themselves in the game world and telling the gamemaster what they want to do.
Dice-Neutral -- Rules examples use six-sided dice, but tables allow substitutions: 1d20, 3d6, roll over target, roll under target.
Modular -- Procedures like combat, social interaction, or advancement are independent, allowing easy replacement. Expansions are also independent and easy to add.
Open -- Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) license.

No actual rulebook is planned. Liber Zero is primarily meant as a reference for adventure modules and a base on which to build expansions. However, there will be a series of reference sheets for players and gamemasters, downloadable individually. So far, this is in the form of three-panel pamphlets.

Gamemasters can give players just the three or four pamphlets they need to guide them through making a character, a much simpler approach than having them read a book. There will probably be a small player's handbook collecting player reference sheets in one place, for those players who prefer examining all the options at once.