The Big Red Button is technically a keyboard with a single button. What that button is, is up to the implementation. It can be an actual keyboard key, a
Turn your ProMicro into a USB Keyboard/Mouse - SparkFun Electronics
A Big Red Button Acting as a Keyboard Using Arduino
Arduino Membrane Keypad Tutorial
Button matrix and SX1509 wiring - Project Guidance - Arduino Forum
Arduino Based 3D Printed Apollo DSKY (X-post KerbalSpaceProgram) : r/arduino
I made a mousejiggler that keeps windows awake and preserves the online status of teams. The computer recognizes it as a keyboard so it is completely undetectable. : r/arduino
JoyToKey with USB Buttons – RasterWeb!
Building your own keyboard (from scratch), by w4ilun
Arduino UNO FAQ, Arduino Tips, Tricks, and Techniques
PC verification using Arduino as USB keyboard - Topic 22864 - TASVideos