Microlite 20

Microlite20 is a simplified version of the D20 SRD; So simplified, in fact, that the core rules are less than 1100 words long.

The rules, while simple, are incredibly modular; They're designed to work with near any D20 supplement, and adaptions of D20 Modern and Call of Cthulhu (tabletop game) have been made, along with basic rules for a Resident Evil campaign, Star Wars, Westerns, and several others have been made, and more are in the works, including Gamma World and the Conan the Barbarian RPG.

The main site for it was here, before the creator lost interest in the project and handed it to somebody else. The new project maintainer has an existing forum community, but the best place to find the pdfs would be here.

Bruce Blaugh has reviewed it.

All in all? It's awesome, and anyone who likes the D20 system should definitely take a look.