Matt Cruikshank has a long, passionate article request in the Suggestion Pot. Hop over and read it in full if you like. One element I’ll seize on is discussing open source/creative commons games.
John Kim maintains links to a vast number of free RPGs . Among them are a subset of open and licensed games, conveniently gathered together as open gaming examples . Eleven open games are highlighted at that link, and many more exist out in the wild. (For example, one of my favorite systems is FATE, a FUDGE derivative. It was released under the OGL and exists on a well organized wiki . One specific, non-public form of the system is Spirit of the Century.)
The Shadow of Yesterday
I enjoyed reading The Shadow of Yesterday  (released under creative commons)– it is a fantasy RPG, but its focus is a little off from the traditional. The game style encourages character interaction and non-standard skill use. The world of near is a fantasy world, but it doesn’t feature the endless monsters you may be used to from D&D. A key motivator for the system are Keys, the form of advancement in the game. Some keys mimic traditional experience (the Key of Bloodlust  rewards overcoming opponents in battle), but others offer very different motivation (like the Key of Unrequited Love ). So far, I haven’t played the game– does anyone have experience with it in play?
The Shadow of Yesterday, linked above, uses FUDGE dice and elements of Fudge’s resolution system. Several gnomes are big fans of FUDGE– Patrick Benson’s review of the system  is thorough. After reading the article you’ll know whether you want to investigate further.
Supplements for FUDGE are everywhere, from Agyris – World of the Bell  and FATE Pendragon , each freely available on the web, to published games like The Deryni RPG  and The Collectors: The Burning House .
D20 and OGL
Probably the most expansive source of ready to mix freely available and usable game information. Several products and websites have tried to organize and highlight great game elements. I liked The Year’s Best d20— a product that really highlighted several good open source options and expansions for D&D. A current project that uses a web community is the The Grand OGL wiki . It organizes extensive excerpts from many many sources– contributing to this project might just fit the bill. It is OGL, which may not be as easy to use as creative commons– but the depth of sources can’t be matched. Many System Resource Documents  are compiled by John Kim, exposing more than a dozen takes on the d20 system.
JAGS is a system I’ve heard good things about. It sounds similar to GURPS in system complexity. It is well supported and the core system is freely released under the GNU FDL. Does anyone have experience with the system? Tell Matt and me about it in comments.
The Wushu open rules  are released under a creative comments license. I have heard about many fun game sessions of Wushu– the system is demanding, but supports wild and over the top action. If cinematic and rules light are your guiding lights, this might be a great system to build on.
Other Deserving Systems
Several other systems are listed on John’s page, and there are many many more systems out there on the web. Is there a system that you think Matt should look into? What system have I unfairly overlooked?
Games Not Officially Open
For many smaller game systems, obscurity is the enemy. While it is always best to check with the author before you sink in too much effort, many game designers and authors would love to have you help them out by providing tools, quick references, and all of the other goodies that you mentioned. Many are very approachable– if you’d like to help make their system more convenient or breathe new popularity into it, you’ll probably find an ally in the author. Jot the author a quick email and mention your ideas. Few will turn away free work and advertising.
In the end, I’m one gnome– while I ready and enjoy a roleplaying games, I haven’t really played that many open systems. Share your ideas in comments. Let us know about games that are ripe for Matt’s efforts or open licensed games that excite you. The web’s an awfully big haystack…