I'd be willing to GM, though I'd prefer it on a rotating basis so I could also play. In terms of systems ...
L5R I've run before, and could run again.
Scion is probably my favourite setting to play in.
Dresden Files is good.
Exalted, though I'd probably wait for 3rd edition, since it's not all that far off.
Any of FFGs 40k line, though Black Crusade tops the list (followed by Dark Heresy, Rogue Trader, Deathwatch, Only War).
Qin and Ragnarok (mythic roleplaying in China / Norse settings) interest me, and while I've skimmed them, I've never properly played them.
Changeling the Dreaming is a game I always wanted to have a try at, but never did. I don't know it well enough to run it, but I would like to play.
When Into The Far West finally makes it out, I'd enjoy a spin at that (Wuxia Western Steampunk)
I played in a Supers campaign using the Smallville rules set, which worked quite well.
I'd consider playing other systems, depending on what I knew of them.