I have to agree on the metal for PKing, but in general I listen to Riverdance/Celtic stuff, I play WoT muds, and I was listening to riverdance when I first read the series, and so I tend to associate the two now.

Edith Piaff works wonders with my inspiration and creativity. I code and dont play. The few times that I do play.... I prefer hack and slash.

I rarely listen to music to "get myself in the mood" or whatever, because usually I spend most of the free time at home mudding =p
Usually I pk on mud, and listen to rock/metal things, mostly 'tallica/manson/disturbed/tool... Though one of the more original things of linking music to mudding I've made was like... There's an area on the mud I play, with a couple of big mobs before the main mob of the area, and the trick is that you have to kill all the smaller bastards before you get to the boss; so I've had a special playlist made, which was kind of timed with the duration of run - I'd have different tunes playing at different mobs, and if we weren't exactly making it in time, I'd hear it instatly by having wrong tune playing at wrong mob =p