QMK (Quantum Mechanical Keyboard) is an open source community centered around developing computer input devices. Early on the community was keyboard focused, but has now grown to include mice and MIDI devices as well. The community maintains QMK Firmware, QMK Configurator, QMK Toolbox, qmk.fm, and this documentation.
Totally new to QMK? There are two ways to get started:
QMK has lots of features to explore, and a good deal of reference documentation to dig through. Most features are taken advantage of by modifying your keymap, and changing the keycodes.