I have a confession to make. While I do often listen to Mozart, Beethoven, or Mussorgsky while I'm working, I've been in the habit recently of listening to video game music. Maybe it's odd, I'll admit, but I enjoy it. In general, it's got no words, and is thus less distracting than if I were to listen to the Barenaked Ladies.
So let me share with you some of my favorite game soundtracks and where I got them. First, I picked up a copy of the Beyond Good and Evil soundtrack at a site called Storm Effect. You can download the whole soundtrack here. I'm fairly sure the game makers released it publicly, and this is a mirror. So I'm pretty sure it's legal.
Ditto the Bioshock soundtrack, which you can download here directly from the 2K Games website. It's a bit more jarring, and I don't listen to it while working as often, but it's some good music.
BlueLaguna is a site with some good soundtracks too. You can get the entire soundtrack to The Legend of Zelda: Twilight Princess off their website here, although I've noticed a windows 'ping' noise at 2:15 in the track Dark Lord Ganon. You can also get one of the best game soundtracks I've ever heard from BlueLaguna: the soundtrack to Castlevania: Symphony of the Night. It's fantastic. Listen to Lost Paintings, Dance Of Pales or Tower of Mist and you'll see what I mean. The Harpsicord in Wood Carving Partita and the haunting voices in Requiem For The Gods are likewise excellent.
It's too bad more game designers don't release their games' soundtracks online. I'd love to get a copy of the soundtrack to Shadow of the Colossus. And while it was a terrible game, the music in Advent Rising was amazing.