Beyond Compare - The file compare and directory compare utility from Scooter Software. I don't understand why file synchronization has to be so complicated. This Beyond Compare thing is nerdy, but it sure is straightforward.
Most depressing thing is that the iPod doesn't expose the files, so you have to use iTunes to synchronize. That means you can only sync with one PC. What you really want is to be able to sync a bunch of machines together.
Beyond Compare does this.