While you hopefully don't need one-button access to 911, a desk-mounted panic button is undoubtedly cool, and it makes for a great DIY Arduino project ...

The 486 that [someyob] is restoring had the Turbo button, but sadly there was just a simple LED to show whether or not it was engaged.

Thorin Klosowski DIY musical instruments are their own thing entirely, but if you're looking for a place to start, Adafruit has a guide to building your own MIDI controller using an Arduino.