qmk/qmk_firmware

QMK Firmware is open-source software that runs directly on the microcontroller inside a mechanical keyboard. Firmware is the low-level program that tells a device's chip what to do, in this case, translating physical key presses into signals that your computer understands. QMK gives keyboard enthusiasts and hobbyists full control over how their keyboard behaves, far beyond what any driver or software running on the computer itself can achieve. With QMK you can remap any key to any function, create multiple layers (so the same physical key does different things depending on which layer is active, similar to a Shift key but fully customizable), program macros that type entire strings with a single keypress, enable tap-vs-hold behavior (where tapping a key types a letter but holding it acts as a modifier like Ctrl), and much more. Because the logic lives in the keyboard's own chip, none of this requires any software running on the computer. The firmware is written in C and targets Atmel AVR and ARM-based microcontrollers, the small chips commonly found in custom and enthusiast keyboards. It officially supports a number of specific keyboards including the Planck, Preonic, ErgoDox EZ, and Clueboard product lines, and has community-contributed support for a large number of other boards. You would use QMK if you own a compatible keyboard and want to customize its behavior at a deep level, or if you are building your own custom keyboard from scratch and need firmware to run on it. It is the dominant open-source firmware in the mechanical keyboard community.