104-key Windows keyboards have this key with a sort of context-menu icon on it, which on Windows simulates a right-mouse-button-press. I'd like to make it act as a right-hand-site 'Option' key instead. Is there a tool somewhere that would allow me to do this?
Look for a mac driver for your keyboard. I use a Microsoft wireless keyboard and mouse. Microsoft provides an additional panel for the System Preferences app which lets me assign functions to the many, many extra keys on my keyboard.
For this specific option for a Microsoft keyboard and using the Microsoft preferences panel, click on the "Options" sub-pane and check the "Reassign Application key" checkbox. This turns the context menu button into an additional option button.
Check out DoubleCommand. It allows you to remap the modifier keys quite extensively.
I think this blog post over at macosxhints is as close as you will get :) Keyboard mapping seems to be pretty complicated in OS X.