I have all my pinball/arcade machines keys on key-chains. They are cheap plastic ones that have a color printed photo of the pinball backglass on it. I have them all hooked on a board that has Asteroids Deluxe side art on it (I removed the art off of an Asteroids Deluxe cabinet and replaced it with new Phoenix Arcade side art) I used the old Asteroids Deluxe art to cover a piece of hardboard for holding all my keys. The board looks great and I can find all my keys quickly. The board is placed behind my bar for easy access.
And of course, I have one "Master" key that fits all of my machines.