The x86-bare-metal-examples project:
https://github.com/cirosantilli/x86-bare-metal-examples
This is a new collection of of “Hello world programs that run without an operating system. Keywords: bare bones, boot sector, MBR, BIOS, UEFI, VGA, Multiboot, QEMU.”
It is a new project, but there’s already multiple programs. The readme has links to multiple information sources.