
The iPod spawned a massive side market in accessories; these days, the same thing is going on with iTunes, as developers add artificial playlist creators that purport to help you rediscover the music in your iTunes library.
One of the latest entries is MoodShuffle, which understands where iTunes puts your music, and how it stores song metadata. Hear a song you like, and you do nothing, and hit fast forward when you hear something you don't like. By doing those things alone, you'll evidently be telling MoodShuffle enough about your mood for it to be able to select the right songs.
Right now, the program is for OS X only, but a Windows version is apparently on the way. Here's a description from MoodShuffle and a screenshot of the app's simple interface:
