Marlin is a firmware for RepRap 3D printers optimized for both 8 and 32 bit microcontrollers. Marlin supports all common platforms. Many commercial 3D printers come with Marlin installed. Check with your vendor if you need source code for your specific machine.
Configurations for Marlin Firmware
Marlin Firmware Documentation Project
Visual Studio Code extension to more easily build Marlin Firmware