From Mac Guides
Senuti is a shareware program that can be used to copy music from an iPod onto your computer, and can add it to iTunes automatically. A common use is if you need to recover your music from your iPod if your computer hard drive gets damaged. Senuti also allows recovery of music from the iPod Touch and iPhone which aren't able to mount as a normal hard drive on a Mac. Unlike iTunes 7 Senuti also allows the recovery of all your music including songs that haven't been purchased from the iTunes Store.
Using the Program
When the program initially loads it runs a setup wizard to guide you through the process of setting up the application, the steps are fairly self evident, make sure to check the "Automatically add to iTunes Library" if you want to do that. Note that in this setup process no actual copying will occur.
Once this is complete the application loads its main screen. Playlists on the iPod appear on the left and songs on the right, songs that are already in your iTunes library will have a blue dot by them. You then press the transfer button in the toolbar to move any songs over to your computer.
Note that with the iPod Touch you may have to disconnect and reconnect it to your computer with the program running for it to appear.
Disclamer: this application should only be used for copying music for which you own the rights.