Coreboot

Coreboot, formerly known as LinuxBIOS, is a software project aimed at replacing proprietary booting firmware (the BIOS or UEFI) found in most computers, with lightweight freeware designed to perform only the minimum number of tasks required to load and run a modern 32-bit or 64-bit operating system. In most cases it is required to disassemble the machine to access and flash it's BIOS chip with the firmware, which consists of two parts, the actual coreboot code, which has parameters that need to be set, and a payload which coreboot hands control to after initializing hardware. Many builds of coreboot exist, most notably from the Libreboot project, which adds a few optimizations on top of the coreboot code and which are automatically built by Libreboot's build system, lbmk. Since coreboot initializes bare hardware, it must be ported to every chip-set and motherboard that it supports. As a result, coreboot is available only for a limited number of hardware platforms and motherboard models. Devices supported by Libreboot are even more limited, as Libreboot prides itself on not using any proprietary blobs. This however has changed recently (add more here).

You may have heard the myth that unlike libreboot Coreboot contains non-free blobs, but this is not entirely correct. The trannies have lied to you. Coreboot comes without blobs by default on machines that don't need blobs, and only comes with the blobs required on machines requiring them. This allows it to support a large number of motherboards. Libreboot was created because the fsf hates everything that even gives you the option (the freedom) to install proprietary software. Coreboot is just as free as Libreboot on machines that support both. (rews)

Payloads

Coreboot by default use the simple as fuck SeaBIOS payload. Which is probably the closest thing to a stock BIOS. This payload supports a custom splash image. You can get some here They have to be saved in a weird way in GIMP[1]

The other payload that is generally used is GRUB, this is used to have Full Disk Encryption. This is the payload libreboot use for their most releases.

Moreover, there is the EDK2[2] payload which implements [UEFI]. You can even have Linux in flash as a payload (and boot Linux off of disk using kexec).