Marlin is a straightforward, Open Source firmware designed for the RepRap family of 3D printers.

It is reliable, extensible, and adaptable, making it the go-to for commercial 3D printer vendors and hobbyists. Marlin runs on inexpensive 8-bit Atmel AVR microcontrollers. It additionally includes the added support for 32-bit and ARM-based boards. Most Marlin builds are generally small enough for use on a headless printer with only modest hardware, and features can be enabled as required. That makes it an excellent choice for 3D printing enthusiasts who don't have an unlimited budget to satisfy their hobby needs.

Marlin Firmware runs on the 3D printer’s mainboard, managing all the real-time activities of the machine. It coordinates the heaters, steppers, sensors, lights, LCD display, buttons, and everything else involved in the 3D printing process. There is an extensive introduction section available at the author's site to get you started.

Marlin Features:

Full-featured G-code with over 150 commands
Complete G-code movement suite, including lines, arcs, and Bézier curves
Smart motion system with lookahead, interrupt-based movement, linear acceleration
Support for Cartesian, Delta, SCARA, and Core/H-Bot kinematics
Closed-loop PID heater control with auto-tuning, thermal protection, safety cutoff
Support for up to 5 extruders plus a heated print bed
LCD Controller UI with more than 30 language translations
Host-based and SD Card printing with autostart
Bed Leveling Compensation — with or without a bed probe
Linear Advance for pressure-based extrusion
Support for Volumetric extrusion
Support for mixing and multi-extruders (Cyclops, Chimera, Diamond)
Support for Filament Runout/Width Sensors
Print Job Timer and Print Counter