I'm a moody FKR...(No need to agree so whole heartedly!). As such I like to listen to different types of music depending on my mood (surprisingly not always Suicide Metal [yes it is an actual genre]!). To do this I have in the past created a number of special 'play lists' and custom searches. Well, no more...
It's a free application that integrates with iTunes and allows you to tag your music collections according to mood. You do have to go through your library and pick a colour based on a scale between happy and sad and also calm to intense. After that you can then choose a particular mood tag, such as "Intensely sad", "Happy Calm", "Intensely Happy", and get an accordingly populated play-list.