When I was a kid, I loved the opening theme song [http://www.youtube.com/watch?v=B0CYNm_dRBA&feature=related] for the Saturday morning cartoon series of Super Mario World (the one with Yoshi and cavemen and stuff). I had an episode on tape and I would just play the theme song part over and over again.
I don't actually watch the show, so I'm not sure if it's the theme or not, but on ads for the new season of The Deadliest Catch, there's this really beautiful, mournful violin tune. And the original theme song, Dead or Alive, is pretty cool too, actually.
And I agree about Firefly and Futurama, great tunes.