All sorts of things - gaming OSTs, Tom Waits, Mark Lanegan, anime openings, Swedish punk music, etc.
But mostly, I listen to history-podcasts. I find it's easier to focus when I'm listening to longer stuff - like, 20-30 minutes. It keeps me working through the whole podcast episode, so I get more stuff done. With music, there are pauses between songs every 3-5 minutes, and it makes my brain wander off and lose focus.
Right now I'm listening to The British History Podcast, Mike Duncan's Revolutions (and before that, his History of Rome), The History of Byzantium, The History of Philosophy and The History of the Crusades. I focus better, get work done, AND learn cool history things all at once!