The CORE Codex

From the introduction:

The Core Codex contains a set of rules and guidelines called the Character Oriented Role-playing Engine (C.O.R.E). The bulk of the rules have been curated from almost twenty years of role-playing experience and built around a d20 System base. Over the last few years these rules have been modified, prodded, twisted, kept in a vat for genetic experimentation and finally boiled in a magical cauldron to be delivered to you the reader. The Core rules is an “engine” intended to power any kind of role-playing game (‘RPG’). While the system is written as a balanced set of rules, they should be viewed more as a set of building blocks and guidelines allowing players to create their own role-playing games.