iTunes makes it really easy to put music from your computer onto your iPod--unfortunately, the reverse is not true. If you want to get music from your iPod onto your computer--say, if you had a hard-drive failure, or you just bought a new Mac--you either need to know how to use Terminal or you need an app like Senuti.
This cleverly named utility essentially does what iTunes does backwards, acting as an "iPod ripper" that lets you move your music, playlists, videos, everything, onto your Mac. Senuti has been around for a few years, but it's still sub-1.0, plus it's free--so be warned that the interface is very simple and you won't get a lot of hand-holding here.
Fortunately, the online documentation is good, and if you take just a few minutes to read the short QuickStart guide and FAQ, you'll save yourself time and avoid trouble like creating duplicate tracks or inadvertently letting iTunes erase your iPod. Until El Jobso puts computing before the RIAA, Senuti is an efficient way to get around a frustrating problem.