When it comes to creativity and problem solving at the development level, I find that like others here, either no music or classical music on low volume assists the concentration. Beethoven, Bach, and Mozart, etc. I truly find classical helpful... I read once that it actually stimulates the proper receptors in the brain that involve critical thinking, but I'm unable to find that link as a source. I stumbled upon classical years ago at random I think... I never liked it before then and I'm certain for some it can be an acquired taste.
(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.
It may just be a psychological placebo but I seem to find my groove much easier when I'm uninterrupted and listening to music that blocks distractions.
And given the number of other people who express similar tastes it makes me think there may be a market opportunity for a team of music theory nerds + neuroscientists + programmers to come up with a box that generates "music to think by".
(I want to believe that certain Bach fugues are wired into the universe at the level of apodeictic mathematical truth.)
For some reason, Godspeed You! Black Emperor really jives with me when I am working on hard problems (To be fair, the problems I work are not as impressive as most of you here)
Occasionally I'll listen to a Tamboura meditation - which is just droning - lower intensity than the club tunes, but similar effect.
Now excuse me while I put on some Armored Saint...
At a secondary level, some music seems to act a little like stimulants. But that is secondary to drowning out conversations.
The more upbeat and progressive the better. Too much volume ruins my flow though.
My day is shot when I forget to charge my iPod...
probably varies depending on the person as well.