I'm using an old Apple USB keyboard(it came with the iMac G5) with my Mac. I'd like to bind F16 to play/pause iTunes, like F8 key on the MacBooks. Right now the key does nothing.
I'm OK with 3rd party software/plugins.
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The best solution for this is Karabiner (formerly known as KeyRemap4MacBook).
It allows you to do all sorts of great remapping, including this one. It isn't the easiest thing in the world to set up though. Using it and writing your own custom keybindings file almost anything is possible.
For a commercial solution that also does a great deal more and is easier to deal with you could use Keyboard Maestro, which I consider almost essential on a Mac.