When Apple introduced iTunes 4.9, they also introduced a rather annoying bug where a valid MP3 occasionally gets identified as a "QuickTime Movie." The problem comes when you try to sync to your iPod, because iTunes refuses to copy the songs over. iTunes MP3 Rescue to the, um, rescue!
When you launch the program, it will search your iTunes library for MP3 files that are incorrectly identified and repair them. Simple as that.
There's actually a good chance that Apple has fixed the bug in iTunes 5.0, but I'm providing this in case they haven't.