U-Boot

Full name Das U-Boot, a pun on the German submarine film. It began as PPCBoot for PowerPC boards, was long maintained by Wolfgang Denk at DENX in Germany, and got its new name as it spread to ARM, MIPS and later RISC-V. It brings up RAM and clocks, loads the kernel from flash, SD card or the network over TFTP, and hands over along with a device tree describing the hardware.

Hook a serial cable up to a router and you'll often see a => prompt behind an interruptible countdown. From there you can read and write memory, change the bootcmd environment variable and boot whatever you like — which makes that prompt both the developer's lifeline and the hardware hacker's favourite door. Recent versions can also act as a UEFI environment and verify signed images.