I was playing around today with an app called Moody by Crayon Room. It is an iTunes add on that allows you to tag songs based on emotional criteria. Songs are tagged using a colored grid to indicate how happy or sad, calm or intense your find them. Songs can then be played back using one, or multiple mood tags, essentially creating a mood specific playlist.
I like this idea because I do listen to different music depending on what mood I'm in, but I wish there was a more automated way to create the tags. I appreciate that adding the tags yourself makes them personalized (as in, a song that makes me happy could make another person sad), but I don't have time to tag all 2500 songs in my library. It would be better if songs were automatically tagged based on variables such as song speed, genera, etc, and then I could go back and adjust them later.
I was also really intrigued by the idea of using colors to represent moods, as opposed to smiley faces or some other method. It takes a little while to get your head around which colors represent which moods, but I think it works. (Thanks James!)