Here’s a great How-To on turning an old keyboard into a breakout board to use for projects. I can see this being really useful in a MAME build or as a control interface for a uC project if you wanted to avoid buying an I-PAC or something similar.
Note: After doing some reading, this wouldn’t actually be ideal for a MAME build. Keyboards have issues with ‘Blocking’, meaning that you can’t always press multiple keys simultaneously. A device like the aforementioned I-PAC solves this by scanning each key input pin individually, allowing you to press as many keys at once as necessary.