Although you may not see PS/2 keyboards & mice as often on desktop computers, there's still tons of them out there, and they're really easy to interface to a microcontroller. PS/2 devices are powered by 5V and use two data pins to communicate back and forth. There's tons of example code out there for every kind of microcontoller (including Arduinos) for keyboard and mouse interfacing.
So all you need to do is connect from the PS/2 plug to your breadboard. Unfortunately, these plugs are not breadboard friendly. That's why they really dig this connector. It's sturdy and easy to panel mount, and has pre-soldered wires connected to the 6 contacts (of which 4 are used). Adafruit thought it very nice and easy to use!
For PS/2 devices:
- Green wire is +5V
- Black wire is Ground
- Brown wire is Data
- Yellow wire is Clock
- White wire is sometimes Mouse Data for 2-in-1 splitter cables
- Red wire is sometimes Mouse Clock for 2-in-1 splitter cables
Adafruit show it plug into a breadboard, but depending on your breadboard, the wires may be too thin. If you're having connection issues, try soldering a thicker (22 AWG) wire to the little wires before plugging them into a socket or breadboard.