The Wire. Seasons 1 and 4 especially.
short and to the point, usually kicks in after some fucked up opening scene
cant find the actual opening of Justified on youtube for some reason, but this is the song anyway.
still my favorite 80s cartoon intro (LOL just noticed this is the spanish version...Thundercats, LOS FELINOS COSMICOS!), followed closely by G1 Transformers
Also the True Blood one, but people already linked it
A lot of my favorites have already been mentioned but there are still two that haven't.
All versions of the Fringe them are fantastic. I like that its always the same theme but changes slightly to fit the current story arc. The 1985 theme in particular is genius.
And I can't believe no one has put up the X-Files yet. Its a classic.
@Aegon: I think this whole fear of youtube spam is getting a little ridiculous. So much so that there was a post a few days ago where someone with decent topic and a well constructed top post was so afraid of getting locked he opted not to put a video in that would have illustrated the point, and then the second or third poster put that video in for him to make it "ok". Shit like that is ridiculous. I know the forums have rules, but there is a difference between youtube spam and using videos and/or having a video in your post. This thread has over 70 posts and people are having a good time, actual spam would not garner such a response. Posting a video does automatically make a thread youtube spam.
That aside, here is my favorite TV theme:
Deny it if you must but sometime during your childhood you thought the Mighty Morphin' Power Rangers theme was AMAZING.
Also, I'll second the Venture Bros. opening music. J.G. Thirlwell makes some amazing music.
I'll third it.
Best use of a licensed song ever. What a way to end Season 4.