Free Systems

Alpha-Zulu Games
Contains to free games: Alternacy, featuring character-driven skill development; and Mnemonic, a minimal rules, memorizable system designed for portability.
http://www.alphazulugames.com/

Alternate Realities
A free universal RPG by Carter Butts, Karim Nassar, and Brian Rayburn. Approx. 88 pages (HTML, MS Word, Postscript).
http://www.karimnassar.com/AR/

ARM
Minimalist system using logarithmic scales for all game mechanics.
ftp://ftp.trawna.com/pub/rpg/games/arm

The Awakenings Rules Engine
TARE, a universal rules engine. The complete basic rules are currently free, with two commercial campaign settings available for it.
http://www.geocities.com/tare_rpg/

Campaign HQ
By Craig Griswold. Contains scenarios, rules, and paper minis.
http://www.pcisys.net/~pandemonium/effective/

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.
http://www.fantasylibrary.com/lounge/cosmic/cosmic.htm

DCS
A simple role-playing engine, which can be customised for various game worlds and game types.
http://www.geocities.com/Area51/Vault/4947/dcshome.html

Draft RPG
A small set of rules suitable for running campaigns of independent backgrounds. Open content license, downloadable in PDF format.
http://www.ekkaia.org/rpg/draft/

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.
http://www.malaan.com/dramatis/dramatis.htm

DUDE Role-Playing Game
This diceless universal determination engine is a one-stat, cinematic, freeform RPG, with several sourcebooks.
http://kuoi.asui.uidaho.edu/~kamikaze/DUDE/

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.
http://red-irish.itgo.com/Games.html

Epitome - Continuous Roleplaying Game
Freeform organic diceless system that puts system mechanics in the hands of the users.
http://www.ekkaia.org/rpg/epitome

Ex Mortis V 1.0 BRPS
Ex Mortis base role playing system. Free to the public. Fully functional RPG.
http://www.angelfire.com/games3/exmortis/

Fantastic Realities
Works with any time period and genre, includes 16 species, magic and monster systems, and realistic rules all of which are optional.
http://jeremy.gagliardi.com/Real-Playing/

GAME
Skill- and attribute-based rules designed to fit on one page.
http://www.homebrew.net/games/game.html

Generic Roleplaying All Purpose Engine
A free, one page, generic game system.
http://unseelie.org/grape/

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."
http://www.sjgames.com/gurps/lite/

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.
http://www.ekkaia.org/rpg/hexatome/

Hyper
Generic role-playing game. Official site.
http://members.tripod.com/~ynam/hyper/

Impresa Modular Roleplaying System
Modular RPG construction set. Free express rules for download include open collaborative license.
http://www.pigames.net/impresa.php

Infinite Horizons
Freely available rules for a multi-genre role-playing game and supplements.
http://ih.trawna.com/

Kaleidoscope RPG Support Page
An open ended system that adapts to a range of settings and genres with few rules.
http://www.fantasylibrary.com/lounge/kaleidoscope/kaleid.htm

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.
http://www.geocities.com/mithrapolis/ladder.html

3Muse D6 RPG System
Free multigenere RPG system. Currently has fantasy and SuperHeroe rules.
http://3muse.tripod.com/

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.
http://kuoi.asui.uidaho.edu/~kamikaze/Nuelow/

OPIGS:
Simple one page gaming system.
http://www.homebrew.net/games/opigs.html

OPIGS Supers
One page gaming system for super heroes.
http://www.homebrew.net/games/opigs.supers.html

Oroborus multi-genre RPG
Rules light, point allocation, skill based game from Ragnarok Press.
http://www.ragnarokpress.com/oroborus/

The Osprey Open Source Roleplaying Engine
A point-based, skill-based roleplaying game distributed under an Open Content license.
http://home.nc.rr.com/archer7/rpg.html

Percentile Dice Game System
A 2 ten sided dice using system released under the Free Gaming License.
http://www.geocities.com/antikrist669/

Player's Labor-saving Utility System
Alan Glvoer's archive of Joe Coleman's PLUS RPG.
http://www.chorazin.org/jorune/plus/

POLY, a free, customizable system for table top and LARP
A modular system that is adaptable to many styles of play.
http://www.angelfire.com/rpg2/undermine/index.html

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.
http://www.rpglibrary.org/systems/prism/

Quasar Role-Playing Game
Public domain rules intended for family-friendly roleplaying.
http://www.conroyhome.net/alan/quasar/index.htm

Role Playing System
Foundations for a game system, reference implementation of that system and resource page to develop game worlds.
http://yewenyi.tripod.com/

SAGA
A small system using attribute based dice pools.
http://www.homebrew.net/games/saga-1.1.html

Shadows
Simple yet powerful roleplaying for all ages.
http://www.harlekin-maus.com/games/shadows/shadows.html

Simple Rules Engine
A two page generic skill and attribute based roleplaying engine, focused on characters and stories.
http://dmsteve-jp.tripod.com/

SLUG
Simple, Laid-back Universal Game. By Steffan O'Sullivan.
http://www.homebrew.net/games/slug.html

ToAd
A model reality kit, where what seems real is real.
http://www.talesofadventure.net/1whatsnew/

Universal
Features a free, universal roleplaying game system. Also includes supplements for the fantasy and Doctor Who genres.
http://www.angelfire.com/games4/doctorwhoeyespy/index.html

The Window
Flexible, modular rules built around three gaming precepts.
http://www.mimgames.com/window/