If you have any iTMS purchases the transfer of purchased content from the iPod to authorised computers has been introduced with iTunes 7. A paragraph on it has been added to this article::
Transfer iTunes Store purchases using iPod
The transfer of non iTMS content such as songs imported from CD is designed by default to be one way from iTunes to iPod. However there's a manual method of copying songs from your iPod to your Mac. The procedure is a bit involved but if you're interested it's available at this link:
Two-way Street: Moving Music Off the iPod
If you prefer something more automated there are a number of third party utilities that you can use to retrieve music files and playlists from your iPod, this is just a selection. 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. You can also read reviews of some of them here:
Wired News - Rescue Your Stranded Tunes
Senuti Mac Only
iPodRip Mac Only
PodWorks Mac Only
PodView Mac Only
iPodAccess Mac & Windows
YamiPod Mac & Windows
PodUtil Mac & Windows
iPodCopy Mac & Windows
