A very useful application. Recently I had to send my iBook in for service, with a hard drive failure. It came back with a new HD and new OS install, but my only backup of my mp3's existed on my iPod. Without paying attention, I hooked up the iPod so that I could copy them back, and before I knew it I had clicked Yes to the question asking if I wanted to set the iPod up for use with this computer.
Of course, now all my MP3's were gone as the iPod sync'd with the empty music collection on my ibook. Oops! I figured since it only took a few seconds to empty it of 15 gigs of music, it didn't actually delete them but rather reset the database and marked the files for overwriting. I searched for an undelete utility and this was the only thing designed specifically for the iPod and music files.
I'm happy to say it worked great and recovered every single file I had. It was simple to use, and even better it was only $40 whereas every other mac recovery utility I could find was at least $90. They do not offer a demo, but do have a 30 day money back guarantee. The app can also recover MP3 files from any hard drive or other device, so it can come in handy if you accidentally wipe out your MP3 collection on your hard drive. And it claims to be able to deal with an ipod re-initialization or database corruption. As long as you haven't written any new files to the iPod since the delete, it should be able to recover them.