Libreboot is a free BIOS or UEFI replacement (free as in freedom); libre boot firmware that initializes the hardware and starts a bootloader for your operating system. It's also an open source BIOS, but open source fails to promote freedom; please call libreboot free software. Since 14 May 2016, Libreboot is part of the GNU project.
Many people use non-free boot firmware, even if they use GNU/Linux. Non-free BIOS/UEFI firmware often contains backdoors, can be slow and have severe bugs, where you are left helpless at the mercy of the developers; you have no freedom over your computing. By contrast, libreboot joins GNU in building a world where everyone can use, study, adapt and share software, with true control and ownership over their technology. In other words, you should use Libreboot for your freedom's sake!
Libreboot is faster, more secure and more reliable than most non-free firmware, and can provide many advanced features (such as encrypted /boot/, GPG signature checking before booting your kernel, ability to load an OS from the flash chip, and more).
Libreboot's main upstream providers are coreboot (which we deblob, for hardware initialization), depthcharge (bootloader, and default libreboot payload on ARM), and GNU GRUB (bootloader, and default libreboot payload on x86). We also integrate flashrom (for installing libreboot), and several of our own utilities, scripts and configuration files. All of this is integrated into a single, coherent package that is easy to use. We add our own patches to the various upstreams used, and where feasible try to merge upstream as much as possible.
Libreboot provides a fully automated build system and installation process, with documentation written for non-technical users, in an attempt to make the software as easy to use as possible. ROM images are provided, along with utilities, all built from the publicly distributed source code.