Unlike conventional approaches which quantize emotions into classes, [this demo] defines emotions by two continuous variables arousal and valence and employs regression algorithms to predict them. Associated with arousal and valence values (AV values), each music sample becomes a point in the arousal-valence emotion plane, so a user can easily retrieve music samples of certain emotion(s) by specifying a point or a trajectory in the emotion plane.

I’ve always enjoyed sorting iTunes music by mood with Moody.