System One Generic Role Playing System

From the introduction:

The intent and purpose of this game system is to create a well-developed, complete, detailed, playable, realistic, and, most of all, enjoyable role-playing game system. This rulebook is not complete in itself. Indeed, all the rules required for playing the game are found within this document, but what lacks from within is the background material, equipment lists, player character races, et al. This done on purpose. The purpose, unlike many games, is not to rob you of all of your money in purchasing supplements, but rather to allow you to have a complete body of rules to play with, and to allow you to create the background you wish.