usually it's some ridiculously crappy pop song that i accidentally heard in a store that gets stuck in there, and since i have no idea how it continues or who it's by it won't go away. the only way i shake it is to listen to it in its entirety so that the clip in my head goes away.
there's been one stuck in there for a week...but i can't remember it now (it's been in tons of ads, most recently a pantene commercial) i hate it.
since i have itunes on all i have in my head is what's playin - Time and Time Again by Papa Roach, with the Card Cheat by the Clash starting right now.