and there's also DREAM...an organization with fighs in Japan (similar to the now non-existent PRIDE) featuring top talent....the UFC doesn't have all the best fighters...lots of them but certainly not all of them. For example, the #3 LW in the world is Aoki and he fights for DREAM. http://en.wikipedia.org/wiki/DREAM_(Mixed_Martial_Arts)
If you have HDNet, you can watch the DREAM events.
To name a couple more....
IFL which features more of a camp concept with a decent amount of championship fights - each card seems to have three now.
Affliction, a new org, with a show in July - Fedor v. Sylvia, Josh Barnett v. Rizzo, Lindland, Babalu, Rothwell v. Arlovski, Aleks Emelianenko v. Buentello, Nogueira (of the Rodrigo variety). Several of these fighters have fought in the UFC but left for various reasons (ridiculously strict contracts, limited success, etc.)
It's all MMA and it's all good.