Even with Neil Gaiman singing this one, I still maintain that "Supercar," another Gerry Anderson show, had the best theme song of all time.
On a live episode of This American Life, Joss Whedon played the piano and sang a song from Commentary! The Musical.
(Dr. Horrible's Sing-Along Blog)
Not to be outdone by cats, here's a dog singing the "Game of Thrones" theme.
You know, I never appreciated Rent until I heard it in its original Klingon. Quite beautiful, actually.