The Perfect20 System is a variant of the SRD system developed by Levi Kornelson. It is intended as a streamlined version of the SRD System from Wizards of the Coast, similar to (and influenced by) the True20 System by Steve Kenson. Compared to the original SRD, Perfect 20 removes all non-d20 rolls, replaces classes with a set of ten Advancement Traits picked at character creation time, and has fixed track of health levels rather than increasing hit points. It is available in PDF format from his site. I have done a port of it to an editable text form, for use by other open gaming projects.