Man, there are so many.
In Nomine can be pretty cool.
GURPS I have had lots of fun with in the past.
But then there's stuff like Call of Cthulhu.
Mage: The Awakening is so much damn fun. All of the old World of Darkness games that aren't Vampire, Werewolf, or Wraith really get me. I love playing Hunter: The Reckoning, and Changeling: The Dreaming is one of the craziest games you can play. Then there's Exalted, which is like playing DnD if it were World of Darkness and Final Fantasy.
Then there's stuff like Shadowrun, which is really a lot of kitschy fun. Deadlands has a pretty great setting too. Now, recently, I've played a bit of the new World of Darkness game Scion, which is very Neil Gaiman/American Gods-esque. It's a lot of fun. Let's see...
Even though I love a lot of these games dearly, especially the World of Darkness ones because of the immense flexibility the GM is given and the immense roleplaying opportunities, DnD will always be my 'old stomping ground'.