Maybe it's because I'm home alone most of the day, but I always like to have some music on in the background. I discovered Pandora.com a little while ago and I usually have it on when I'm at the computer. It lets you create your own stations based on music you like - very cool.
When I'm writing it always helps me to have a little mood music on. I really like instrumental jazz because it's fun and energizing. But I also love movie soundtracks because they have so much intensity. My favorite composer is John Williams because he did all the classics - Star Wars, Superman, Hook, and Harry Potter. I know the music well enough to tune it out while I'm thinking but it gets me in the mood to be creative.
I also like having music on when I'm reading, but I find it easier to concentrate on my book so it doesn't have to be just instrumental. So I pretty much have the music going all day!
What do you listen to when you read or write?