Limine
Limine is an advanced, portable, multiprotocol bootloader, with support for Linux, the native Limine boot protocol, and multiboot1 and 2.
History
Limine was created as the reference implementation for the stivale boot protocols. The protocols were conceived as a response to the shortcomings of Multiboot. As of Limine 3.x, a new boot protocol, simply called the Limine boot protocol was introduced to supersede stivale 1 and 2. As of Limine 4.x, support for the legacy stivale protocols has been dropped.
It originally only supported x86 BIOS and the stivale1 protocol, but it was later expanded to also support UEFI, the Linux and multiboot protocols, and other architectures such as aarch64.
Supported protocols and filesystems
As mentioned above, Limine supports the Limine boot protocol, alongside Linux's own boot protocol (which means one can boot Linux fully using Limine), multiboot 1 and 2 (allowing it to boot a vast catalogue of hobby OSes and more), and chainloading to allow to indirectly boot unsupported operating systems such as Microsoft Windows.
It supports the FAT12/16/32 and ISO 9660 (used by optical media and hybrid ISO images) filesystems.
Supported Architectures
Currently Limine supports X86, X86-64, aarch64 (64 bit ARM), and riscv64 (64 bit RISC-V). X86 system support is targeted for Pentium Pro (i686) class machines as the minimum.
How to use Limine with your kernel
The Limine Bare Bones article contains a basic tutorial on how to use Limine and its protocol. Furthermore, a GitHub repository containing a simple example template for a 64-bit kernel loaded using Limine can be found in the external links section.