Pandora (they also have great classical music stations, including a nice one of only piano works). If I'm kind of pissed off or cranky, I can get a great stream of shoegazer tunes out of Pandora (just using My Bloody Valentine as the seed group), and if I'm on a coding roll I discovered that my BoA station also plays tones of Ghost in the Shell soundtrack tunes, which somehow gives me intellectual energy! Finally, if I'm totally having a blah day, the ultimate weapon is Stereomood, which has an all-melancholy playlist (with a huge variety of songs that I don't know) that will definitely fit how I feel. I guess I've discovered that often the only way out of a mood is to go deeper in, and music certainly helps me do that. Usually when I listen to mood music at some point it won't appeal to me anymore, and then I know the mood's over!