Keycaps was an application under OS 7/8/9 that showed you a picture of your keyboard, with each letter on the keyboard being represented by the actual character corresponding to the font that was selected. You could also hold down Command, Option, Control and Shift keys in different combinations, and the keyboard would change the characters on the keyboard accordingly... for example, you could hold down the "Option" key, and an accent mark would appear above the 'e' key displayed, showing you that holding down Option and then typing 'e' would produce an e with an accent mark. Or if you held down the Shift key, all the letters would be displayed as uppercase, obviously.
You wouldn't believe the amount of characters available when you hold down different combinations of command/control/option/shift keys. It's very useful when searching for a specific "dingbat" from Zapf Dingbats, as you can just hold down different combinations of keys and see all the "dingbats" available, and which key they correspond to.