My solution has a couple problems, but it will work.
I'm going to assume you are running Windows 7. This might also work for Vista, but I'm not sure.
First, go to the start menu and click Computer on the right-hand side. Click on 'Organize' towards the top-left corner of the window. Then select Folder and Search Options. In that window, click on the View tab and enable "Show hidden files, folders, and drives".
Next, find the main directory for your iPod when it's plugged into your computer.
You should see a Music folder or something similar in your iPod's directory. This has all the music that's on your iPod. I like to copy and paste the folder somewhere on the computer's hard drive, but I'm not sure if you have to. You should just in case.
The problem is that even though the music contains all the album and artist data etc, they are in a jumbled mess of folders, the sound files themselves probably even have random names like ai85je even if it's actually "We Are the Champions". So in the Music folder in your iPod's directory, there might be 12 different folders titled "FE1" "FE2" "FE3" etc and within each of those are a mixed up group of songs.
Now, open iTunes->file->Import folder and select the folders with the iPod music. This will import them to iTunes.
The only way I know of to find a certain song is to import all of them onto iTunes and delete all the copies. It's a pain, but it's a way to get music onto libraries without syncing the iPod.
If anyone has a more convenient method, please post. I'm not good at explaining this method either, it's really simple but when I type it out it looks confusing to me.
Summary:
Find your iPod directory, enable "show hidden files and folders", import folders onto iTunes.