Anything by the Magnetic Fields can get stuck in your head quite easily, and if you listen to the 69 Love Songs album, guaranteed that most of those songs are short enough you'll find one that will drive you nuts.
I find myself humming Nothing Matters When We're Dancing quite a bit. I listened to it for three straight days here awhile ago. The thing is, I love it.
I have been humming Tonight You Belong to Me as heard from the movie The Jerk. I found a version of the song sung by Jonathan Coulton and John Hodgman that's just made of badass, and it's the one I want people to hum on these forums, so here you go.
I do love the songs too, so these earwigs aren't as bad as they could be.