A few systems have also published open, free system reference documents (SRDs).
- Fate (and Fate Accelerated Edition) - soon to include the Toolkit (this was released awhile back)
- GUMSHOE (the system behind Trail of Cthulhu, Esoterrorists, Ashen Stars, and more)
- DramaSystem (featured in the new Hillfolk RPG) - I finally got to play this at BigBadCon
- 13th Age (the Archmage Engine, as they call it)
- WaRP (Wanton Role-Playing) - used in Over the Edge, which I'm not familiar with
If you've been interested in any of these systems, give their SRD a look.
* Evil Hat insists that this is not *the* definitive Toolkit for the Fate System, but merely *a* toolkit.