If you have any iTunes Music Store purchases the transfer of purchased content from the iPod to authorised computers was introduced with iTunes 7. You'll find details in this article:
Copying iTunes Store purchases from your iPod or iPhone to a computer
The transfer of content from sources such as songs imported from CD is designed by default to be one way from iTunes to iPod. However there are a number of third party utilities that you can use to retrieve music files and playlists from your iPod. I use Senuti but have a look at the web pages and documentation for the others too, you'll find that they have varying degrees of functionality and some will transfer movies, videos, photos and games as well. This is just a small selection of what's available, you can read reviews and comparisons of some of them here:
Wired News - Rescue Your Stranded Tunes
Comparison of iPod managers
Senuti Mac Only
PodView Mac Only
PodWorks Mac Only
iPodDisk PPC Mac Only (experimental version available for Intel Macs)
TuneAid Mac only (iPhone and iPod Touch compatible)
iPodRip Mac & Windows
YamiPod Mac & Windows
Music Rescue Mac & Windows
iPod Music Liberator Mac & Windows
iGadget Mac & Windows (iPhone and iPod Touch compatible)
Floola Mac & Windows
iRepo Mac & Windows (iPhone and iPod Touch compatible)
iPod Access Mac & Windows (iPhone and iPod Touch compatible)
TouchCopy Mac & Windows (iPhone and iPod Touch compatible)
There's also a manual method of copying songs from your iPod to a Mac or PC. The procedure is a bit involved and won't recover playlists but if you're interested it's available on page 2 at this link:
Copying Content from your iPod to your Computer - The Definitive Guide
