Joan Baez' "Diamonds and Rust" -- I think of my ex when I hear it, and sing it. Of course, that just puts more feeling into it. The same goes with Dylan's "Don't Think Twice" (although I prefer Joan's cover of the tune).
Other than that, I use music as a magical tool to cause a change of state. I've set up mix sets specifically to work through anger, change from anger to happiness, or to let go of things. I put the songs in specific order for that purpose.
Or I put the MP3 player on shuffle and practice "radio-mancy", seeing if what pops up is what I'm really in the mood for. You'd be amazed how often it works.