coreboot

It began in 1999 as LinuxBIOS at Los Alamos National Laboratory, where Ron Minnich wanted cluster nodes to boot without anyone pressing F1 on a keyboard that wasn't there. It was renamed coreboot in 2008. The design is deliberately thin: initialise the memory controller, clocks and buses, then launch a payload — SeaBIOS for legacy operating systems, TianoCore for UEFI, GRUB, or a Linux kernel directly.

Every Chromebook boots with coreboot, which makes it one of the most widely deployed firmwares nobody has heard of. On modern Intel and AMD platforms, though, bringing up memory still needs binary blobs from the vendor, so "open" has become a matter of degree rather than a yes or no.