Saga: A Universal Role-Playing System
Saga is a free, open-source, universal role-playing system in about 30 pages. It's designed for fast play, ease of use, drama, and strong narrative elements and characterization. It is released under a Creative Commons license, and you can use it in your own published products. Did I mention that it was free?
Note: this book is in beta. While it's reasonably final and has been playtested, some of its contents are still subject to change. It was last updated 6/11/07.