MechKey.Org - All about keyboard

Atmega family:

EasyAVR - EasyAVR is a keyboard firmware and keymapper GUI for custom keyboards based on USB AVRs. It is powerful but also really easy to use.
TMK - A very popular firmware that support Atmega32u4 powered keyboard.
QMK - Base on TMK but support a lot of controller, support RGB, easy to use.
The VIA - QMK based firmware for the view app.
Keyplus - An easy to use, wired and wireless modular keyboard firmware, for both ATXmega family and atmega32u4.
Kaleidoscope - Flexible firmware for Arduino-powered keyboards.
Pipit keyboard - Firmware and config tools for chording keyboards.
rusty-c - QMK in Rust.
QMK-VIA-Hex - Precompile via firmware for qmk supported keyboard.

Raspberry Pi Pico:

RMK - A feature-rich Rust keyboard library with wireless and split support, aims to make keyboard customization easy and accessible.
KMK - Clackety Keyboards Powered by Python.
PicoMK - Configurable firmware for RP2040 powered mechanical keyboard.
Prk firmware - A keyboard firmware platform in PicoRuby.
PYKB - A highly extensible firmware written in Python, based on CircuitPython, with RGB, USB and Bluetooth. similar to TMK/QMK.

STM32 family:

Nordic family:

BlueMicro - A Keyboard Firmware for nRF52832 Boards with a pinout compatible with the Arduino Pro Micro.
Bluetosis - Bluetooth firmware for the Mitosis keyboard.
Lotlab/nrf52-keyboard - TMK based keyboard firmware for nRF52 series, now support both nRF52810 and nRF52832.
Lotlab/nrf51822-keyboard - This is a firmware based on nrf51822 Bluetooth keyboard, which uses nRF SDK 10.0 as the underlying hardware driver, and uses TMK keyboard library as the upper part of the keyboard function.

Esp32 family:

T-vK/ESP32-BLE-Keyboard - This library allows you to make the ESP32 act as a Bluetooth Keyboard and control what it does.
MK32 - BLE keyboard firmware for ESP32 microcontrollers,designed to be relativley easily customizable.
Key To Key - Rust libary for building keyboard firmwares.

Others:

Firmware