What this project lacks in impressiveness, it makes up for in one-big-button-ness. The classic macintosh hasn't ever been called a hacker friendly platform. That's partially where the allure of doing something beyond software comes from.
This is my first step into the world of the Apple Desktop Bus. It's not a big step. The hardware is based on a pic microcontroller. Using Microchip's appnote (AN591), most of the work was already done for me. The appnote was originally for a pic 16C56, so I converted it to run off of a 16F84 since that's what I had on hand.
For the adb cable, I sacrificed a crappy third-party adb mouse.
When you press the button, the adb device sends "command-w" and that front most window bids you adieu...
... or does it? Good question. Some windows don't respond to command-w, so in order for this to be a proper window closer I would need to write a device handler for this adb device.
Source code and hex file are available here. The schematic is in the App Note pdf.