Libreboot - Libre Boot Firmware

The libreboot project provides libre boot firmware that initializes the hardware (e.g. memory controller, CPU, peripherals) on specific Intel/AMD x86 and ARM targets, which then starts a bootloader for your operating system. Linux/BSD are well-supported. Libreboot gives you freedoms that you otherwise can't get with most other boot firmware. It's extremely powerful and configurable for many use cases.

libreboot is a coreboot distribution. If you want to build a ROM image from scratch, you otherwise have to perform expert-level configuration of coreboot, GRUB and whatever other software you need, to prepare the ROM image. With libreboot, you can literally download from Git or a source archive, and run make, and it will build entire ROM images. An automated build system, named lbmk (Libreboot MaKe), builds these ROM images automatically, without any user input or intervention required. 

Coreboot is notoriously difficult to install, due to an overall lack of user-focused documentation and support. Most people will simply give up before attempting to install coreboot. Libreboot's automated build system and user-friendly installation instructions solves this problem.