Firmware

BlueMicro_BLE - A Bluetooth Firmware for the NRF52832.
Bluetosis - Bluetooth firmware for the Mitosis keyboard.
EasyAVR - is a keyboard firmware and keymapper GUI for custom keyboards based on USB AVRs.
Kaleidoscope - Flexible firmware for Arduino-powered keyboards.
Keyberon - A pure rust keyboard firmware.
Keyplus - An easy to use keyboard firmware with support for wireless and wired split keyboards.
KMK - is a feature-rich and beginner-friendly firmware for computer keyboards written and configured in CircuitPython.
Mechy - Arduino compatible, plugin based.
MK32 - Simple BLE keyboard for ESP32
nrf52-keyboard - A ble keyboard firmware using nrf52810/52832
Python-Keyboard - A highly extensible firmware written in Python, based on CircuitPython, with RGB, USB and Bluetooth.
QMK - A keyboard firmware based on the tmk_keyboard firmware for Atmel AVR and ARM controllers.
TMK - Keyboard firmwares for Atmel AVR and Cortex-M.
ZMK - MIT-licensed firmware built on top of the Zephyr RTOS that is “wireless first”.

QMK

kbd-prettier - Automatically format your keyboard layout
Keyboard Firmware Builder - Online QMK Firmware Builder (2+ years behind QMK firmware master)
Q2K - A python based utility for parsing QMK Firmware keymaps to various outputs
QMK Configurator - An open source configurator for QMK Firmware
qmk_emote_macro_generator - Easily generate QMK macro/leader sequences for arbitrary unicode sequences.