How to unlock 5th generation iPod Nano
The first trick was to remember where I put the cable, which is apparently called the "Apple 30-pin cable".

Having the original Mac or iTunes was not necessary.  I plugged the iPod in to a Linux device (although this should also work from any Mac with a Terminal window).  I located where the device was mounted (let's say it was /Volumes/iPod) and then ran the following command in a terminal:

rm /Volumes/iPod/iPod_Control/Device/_locked

I then ejected the device and it was unlocked!