Core Elements is an OGL d20 derived generic game system. It offers a very basic framework for building a game upon, or may be used as written as a barebones rpg.
designer's commentary from game rules:
Core Elements is a simple, comprehensible, and functional distillation of the world’s most popular roleplaying system.
Like previous editions of Core Elements, the entirety of this release is OGC with the exception of the title. Core Elements can be used either as a set of rules to play a game, or as the foundation of a game that you plan to publish. That said, however you choose to use the revised edition of Core Elements, I hope that you enjoy it.