Open Core System

From the publishers website:

The Open Core System (OGL) is derived from the best of today’s Open Gaming Content, creating a new, strong, coherent and flexible role-playing game system - it’s the house rules you’ve been waiting for. The Open Core System is effects-based to help you create that “perfect” character using attributes, skills, abilities, and disabilities. The Open Core System is “classless,” a point-based system that can allow players and GMs to create a wide variety of characters and world. The Open Core System can handle a wide spectrum of genres “right out of the box,” from science fiction to super-heroes to anthropomorphics to classic fantasy. The Open Core System uses a simple, time-proven base mechanic: 3d6 + Attribute + Skill VS. Target Number. Variant rules for other dice (1d20, etc.) are included. It’s easy to convert characters and campaigns from other popular open gaming systems to the Open Core System. The Open Core System is fully extensible; Open Content from other systems can easily integrate into the Open Core System.