Hurd microkernel
Efforts to get this alternate kernel into a usable state continue.
web | project (other repos) | wikipedia | fsd?Hurd | debian port | GPLv2+

Linux monolithic kernel
You may have heard of it. Contains proprietary blobs.
web | GPLv2-only

Linux-libre monolithic kernel
A free replacement for the Linux kernel. All the blobs have been washed out.
web | GPLv2-only

BIOS and firmware updates

Backlinks: about:copying:switch-question

CC0 / Public domain dedication To the extent possible under law, d3vid seaward has waived all copyright and related or neighboring rights to "Kernel in Grasmere notebook, including code snippets" (why? how?)