When it comes to inspiration for art, musics always been a big part of it -so far going as it actually influences the quality of my work. For that reason, I usually use slow, calming or at the very least melancholy tracks - gives me that little boost to my work ethic, which is sorely in need.
One of my favourites has to be Return to Slumber from the Demon Soul's OST - a great song with some vocals, but not too much. Another one, recently discovered, would have to be Track Rain from the Halo OSDT OST - again great calming song with a little melancholy, but this time no vocals, which is perfect.
However, I usually find myself listening to a rain ambiance song - such as "Sounds of Rain and Thunder on a River" - I've always found rain relaxing, and it's hard to care about getting a piece done, when there's no worry or rush in the song - it really does make a huge difference to how long I spend on a piece at any one time.
Anyway, that's just me. All links are to Youtube, though I've received (bought or dled through Youtube in the case of the rain ambiance) all the tracks at one point or another.