I have seen quite a few applications, etc. that transfer music from your iPod to your Mac. Rather use an application, I have found the following find command to do the same thing. There are two prerequisites you need. Both are defaults so you should have to do nothing. One, all the file formats searched for must be set to be opened with iTunes. Two, in iTunes' Preferences under the Advanced tab, you should have the check box "Copy files to iTunes Music folder when adding to library" checked. Like I said, these are defaults. So you should be OK to run the following find command:

find /Volumes/iPod_name -type f \( -name \*.mp3 -o -name \*.m4a -o \
-name \*.m4b -o -name \*.m4p -o -name \*.wav -o -name \*.aiff -o \
-name \*.aif -o -name \*.aa \) -exec open "{}" \;

Replace iPod_name with the name of your iPod in the above command.

[robg adds: I haven't tested this one.]