Open-source firmware
Open-source firmware is firmware that is published under an open-source license. It can be contrasted with proprietary firmware, which is published under a proprietary license or EULA. Examples 

 OpenWrt
 coreboot
 SeaBIOS
 LinuxBoot
 Libreboot
 Marlin (firmware),  Arduino-based firmware for 3D printers
 PinePhone LTE modem
 Rockbox, a replacement firmware for various digital audio players
 QMK

References

Further reading 

 

Firmware
Open-source firmware