I'm DM'ing a battletech RPG based on the FATE system. You're welcome to use my system. From the standpoint of DM'ing it, the sarna.net wiki is invaluable, as is a basic knowledge of how the battletech board game plays out.

I really like Old School Hack, great for a short campaign with people that have done a lot of gaming

What type of setting are you wanting to play? I can give you a few for which ever type. I'll recommend Fate, Warrior, Rogue and Mage, or even Labyrinth Lord until I can give more accurate systems.

The system that powers Over the Edge just turned OGL and was named Wanton Role-Playing System (WaRP) it's an interesting system.

Check it out here.

Lots of people recommend The Window, I've yet to try a game with it.

I'll go ahead and through Mutants and Masterminds 3e up.. Not all the way free, but you can get two different quickstarts:

Superboy vs Knockout

Batman vs Bane

They also have the D20 srd to get you going.

Basic Fantasy Role Playing Game is a good one if you like Dungeons and Dragons type games you can find it here BFRPG

Thank you for posting the marvel stuff, i am a huge fan of everything marvel and this is like the holy grail.

Already been suggested but Mini-Six is my simple, fast, do anything you want game of choice. Link in sig.

A "Free RPG" thread that goes this far without mentioning Risus is borderline criminal. It's a simple system that really shakes up a lot of your preconceptions about gaming in a way that The Window wishes it could do. (Sorry, I just have a beef against The Window; it's a bit pretentious and condescending to Inferior Games...and a tad hypocritical.)

All things considered, though, Risus is remarkably flexible and brilliant in its simplicity. The more you think about it, the more interesting the system gets. It's 6 pages of RPG brilliance.