If it's code I have to think about (i.e. non-boilerplate), I find that most music distracts me and makes me code slower -- especially music with recognizable lyrics. I have some nature soundtrack CDs that seem to improve my concentration, though.
(I am sure this will be news to a well-below-average number of people on this site, but it's still a novel idea to a lot of people and worth pointing out.)
I've been trying out listening to some sort of white noise, ie. running water, static, etc., borrowing from the idea on this post: http://cameronmoll.com/archives/2008/11/showering_and_thinki...
It can get pretty noisy where I work, so something that distracts me so constantly and blocks the noise out helps me focus.