Alpha-Zulu Games - Contains to free games: Alternacy, featuring character-driven skill development; and Mnemonic, a minimal rules, memorizable system designed for portability.
Alternate Realities - A free universal RPG by Carter Butts, Karim Nassar, and Brian Rayburn. Approx. 88 pages (HTML, MS Word, Postscript).
The Awakenings Rules Engine - TARE, a universal rules engine. The complete basic rules are currently free, with two commercial campaign settings available for it.
Campaign HQ - By Craig Griswold. Contains scenarios, rules, and paper minis.
Cosmic Synchronicity RPG - A free RPG that incorporates concepts of Jungian psychology and Robert Anton Wilson's writings to produce a multi-genre cross-worlds playing environment.
Cyno's Roleplay - A universal RPG by Chieh Cheng. Approx. 33 pages HTML rules, plus four 10-15 page genre books (fantasy, modern, near-future).
DCS - A simple role-playing engine, which can be customised for various game worlds and game types.
Draft RPG - A small set of rules suitable for running campaigns of independent backgrounds. Open content license, downloadable in PDF format.
Dramats Personae - Free simple role-playing system; there are no statistics, little math, no complex charts of equipment, and no magic. Characters represent skilled, but human (or roughly human), beings and have to use their wits rather than their powers, attributes or magic items to succeed.
DUDE Role-Playing Game - This diceless universal determination engine is a one-stat, cinematic, freeform RPG, with several sourcebooks.
EGO-Endless Gaming Options - A simple, universal system, with a stable core rule, a complete foundation, and plenty of room for modifications. Free to all.
Ex Mortis V 1.0 BRPS - Ex Mortis base role playing system. Free to the public. Fully functional RPG.
Fantastic Realities - Works with any time period and genre, includes 16 species, magic and monster systems, and realistic rules all of which are optional.
GAME - Skill- and attribute-based rules designed to fit on one page.
GURPS Lite - Free 32 page PDF version of the GURPS rules. "Covers the essentials of character creation, combat, success rolls, magic, adventuring, and game mastering."
Hexatome: Exploiting the Number of Six - Light-weight role playing game rule system which emphasizes simplicity as well as the scenario and fluently progressing plots.
The Ladder - A generic rules-lite roleplaying game that utilises neat little character descriptions and a five-rung ladder of task results as the core mechanic.
3Muse D6 RPG System - Free multigenere RPG system. Currently has fantasy and SuperHeroe rules.
Nuelow - Series of interlocking games that focus a little more on love (both in the ideal and carnal sense) than those that have proceeded it.
Prism - A versatile multi-genre free RPG by Frank J. Perricone. It is a modular system originally intended for use with Rolemaster's combat system. Over 100 pages, illustrated.
Role Playing System - Foundations for a game system, reference implementation of that system and resource page to develop game worlds.
SAGA - A small system using attribute based dice pools.
Shadows - Simple yet powerful roleplaying for all ages.
Simple Rules Engine - A two page generic skill and attribute based roleplaying engine, focused on characters and stories.
SLUG - Simple, Laid-back Universal Game. By Steffan O'Sullivan.
ToAd - A model reality kit, where what seems real is real.
Universal - Features a free, universal roleplaying game system. Also includes supplements for the fantasy and Doctor Who genres.
The Window - Flexible, modular rules built around three gaming precepts.