Marlin
Optimized firmware for RepRap 3D printers based on Arduino platform
Marlin is a popular open source firmware for the RepRap family of 3D printers. It is straightforward, reliable, extensible and adaptable, which is why it is used by many respected commercial 3D printer vendors as well as hobbyists. It runs on inexpensive 8-bit Atmel AVR micro-controllers, with added support for 32-bit and ARM-based boards. Marlin builds can be small enough for use on a headless printer with only modest hardware, and features can be enabled as-needed.