In case you were having an inferiority complex because your friends' IBM Model M keyboards are so much louder than the shitty rubber dome freebie you got with your pc... Here's the solution: Zeus Hammer, a simple typing cadence enhancer for PS/2 keyboards.
The connects to the keyboard's PS/2 clock line and briefly actuates a large solenoid on each key press. An interesting fact about PS/2 is that the clock line is only active as long as either the host computer or the input device actually want to send data. In case of a keyboard that's the case when a key is pressed or when the host changes the keyboard's LED state, otherwise the clock line is silent. We ignore the LED activity for now as it's generally coupled to key presses. By just triggering an NE555 configured as astable flipflop we can stretch each train of clock pulses to a pulse a few tens of milliseconds long that is enough to actuate the solenoid.
Since PS/2 sends each key press and key release separately this circuit will pulse twice per keystroke. It would be possible to ignore one of them but I figure the added noise just adds to the experience.
Built on a breadboard, the circuit looks like this.
The completed system looks like this.
Since my solenoid did not have a tensioning spring I used a rubber band and some vinyl tape to make an adjustable tensioner. The small orange USB hub serves as an end-stop because I had nothing else of the right shape. The sound and resonance of the thing can be adjusted to taste by moving the end stop, adjusting the tensioning rubber and tuning the excitation duration using the potentiometer. My particular solenoid was a bit slow so I added some pieces of circuit board as shims between the plunger and the case to limit the plunger's travel inside the solenoid core. Here is another video of the thing in action in which I tune and de-tune the mechanical resonance using the potentiometer.