Music can often set the mood, but it can also change it. Mood Traxxer is a streaming music player that works off of your current mood, and lets you try to either change the mood or stick with it. The choices you get for setting your mood are quite humorous, actually.
The site loads up big and bold, and gets right to the point. Describe what your current mood is, and you'll be introduced to a flash-based streaming music player. The site bases your mood on basic mood, your current activity, your "drug" (don't worry; these are really pretty funny), and whether or not you want your mood to change.
With mood descriptions like "Darth Vader on a bad hair day", it's hard not to immediately start having fun with this web app. The drugs section is pretty funny too, with suggested selections ranging from "Absolut Vodka" to "Green Tea". When you're satisfied with your selections, Mood Traxxer will bring you to the music.
Where this site fell just a little short, for me, was music selection. Almost all the songs I heard were some form of house dance mix, or something very similar. I like the idea of having a personalized radio station based on how I'm feeling at the moment, though. It's a refreshing take to the "Music Genome Project" or similar services. Give it a try for yourself, and let us know how Mood Traxxer affected your mood in the comments.