http://discussions.apple.com/thread.jspa?messageID=11073178iTunes will only give you the option to copy your iTunes Store purchases directly from an iPod to the computer, you'll find details in this article: Copying iTunes Store purchases from your iPod or iPhone to a computer
For everything else (music from CDs, other downloads etc) there are a number of third party utilities that you can use to retrieve the music files and playlists from your iPod. You'll find that they have varying degrees of functionality and some will transfer movies, videos, photos, podcasts and games as well. You can read reviews and comparisons of some of them here: