If you've seen Darren Aronofsky's intense, brilliant and horrific Requiem for a Dream, you'll be familiar with the soundtrack, terrifying violins heralding nothing good, performed by the Kronos Quartet.
Chances are, tho', even if you haven't seen it, you'll know the music. One of Clint Mansell's pieces from Requiem, called 'Lux Aeterna', has been remixed and redoubled and reused in any number of trailers and films. I noticed it this evening in a trailer for Sunshine, and it was also in The Da Vinci Code and Zathura.
It's a wonderful piece of music, thoroughly worthy of being reheard often, tho' its regular appearance in other films does seem rather lazy, a way of summoning the fear of Requiem without any of Aronofsky's skill.
It's haunting when just performed by Kronos, but works well with the full orchestra and chorus added for Lord of the Rings: The Two Towers.
Here is the original, used in a fan's recut trailer: