Please keep this about live action shows, leave cartoons and anime series for another thread if someone wants to start one. You don't actually have to like the show itself, just the theme tune.
Mine are:
CSI: NY - Baba O'Riley by the Who (I haven't even seen many episodes of this show, but the song is a true classic)
http://www.youtube.com/watch?v=x2KRpRMSu4g
Twin Peaks - Twin Peaks theme by Angelo Badalamenti (not sure if there's another name for it - a beautiful and haunting melody, I loved this show in the early 90's, forever sad about it ending with a cliffhanger that never got resolved)
http://www.youtube.com/watch?v=Db5ZhxM8VZQ
Honorable mention:
Knight Rider - Knight Rider theme (a sentimental childhood favorite)
http://www.youtube.com/watch?v=nF_rVUbvT3w
What are yours?