I often get songs stuck in my head, but I've noticed a curious thing: I'm especially proned to getting songs with a harmony stuck in my head. It's a puzzle and my brain won't stoup until it's teased apart the primary melody and harmony. Literally it just keeps playing over and over, even as I type this.
This is the song stuck in my head right now. It's a great rendition of Michael Jackson's beat it. I especially love the Xylophone -- it's partially syncopated (plays, sometimes, on the off beat). The harmony starts at 1:14. Warning: it's addictive -- your brain, too, may not stop until it's figured out the harmony!