Imagine this situation:
You computer dies. You don't have a backup of your iTunes folder because you thought you had a back up in your iPod, but when you connect your iPod it only offers to copy back the files that you bought from iTunes. ...what about the 300 CDs you ripped? music you downloaded? podcasts? etc...
Hopefully you didn't say yes when iTunes said 'the iPod is synced with another computer can I wipe it and sync with this computer?'. You can get all that back, it's just not all that obvious. Set up iTunes to allow the iPod to connect for drive access (if it isn't already showing up as a drive)
There is a hidden folder called iPod_Control with all your MP3 files in it, but the files have been re-named to a numbering scheme that helps the iPod keep track of them, but makes it completely non-obvious which file is what to the user who manages to get in this far.
The fortunate thing is iTunes will be able to figure it out.
If you aren't already letting iTunes manage your music you might as well now, but if you'd prefer to have them back with sensible names so you can use another program it's just going to take a few extra steps.
To get these files into iTunes, just copy the lot of them to the hard disk, and open them in iTunes. If you have iTunes manage your music and it's set to automatically import then you are basically done. You just delete them again. iTunes will have already made a copy into it's managed directory. (which you should back up to make this even easier next time).
If you don't have iTunes manage them, then point iTunes to this directory and let it index them all. Now that they show up in the iTunes listing, drag them each from iTunes to a new folder. iTunes will automatically give them each a logical, human readable filename. When you are done you can blow away the random number named versions.
Now that iTunes has your music again you can re-sync your ipod.