[This used to work – Not sure if it is still possible…]
If your iPod can be seen as a hard disk so you browse it on your computer you can drag the numbered folders from the iPod (containing the music) and drop into your new itunes libary (assume only if unprotected) and it will import the music.
You can use 3rd part iPod backup software to copy music off an iPod – I understand 'MediaMonkey' will do this but there are others just google (I've only ever used my drag and drop method but it was some time ago)
p.s. if you have a dvd burner easiest way is, as above, to simply burn your iTunes library/directory to a DVD and then add it back to the new iTunes