At long last there's a second Hurd distribution: Gentoo. During an April Fools post they announced that they were switching away from the Linux kernel because it was "too unstable;" the truth of the matter is that they intend to go forward with support for both systems.
https://www.gentoo.org/news/2026/04/01/gentoo-hurd.html
The Hurd (https://en.wikipedia.org/wiki/GNU_Hurd) is still pretty far from complete from a modern perspective—it has only partial SMP support and the list of supported arches is rather grim. But sometimes shining a flashlight into dark corners is a good way to drive interest in them!
The Hurd is a distant cousin of Darwin, being based around a GPLed fork of the Mach microkernel. It was the 'original' open-source kernel project, pre-dating Linux. Its development history has languished in hell for almost 40 years due to mismanagement, scope creep, politics, and navel-gazing by its contributors, and suffered a severe setback in 2007 after a critique concluded it had several architectural weaknesses. The text of this report (http://walfield.org/papers/200707-walfield-critique-of-the-GNU-Hurd.pdf) has become part of the OS design canon—in the popular mind the Hurd has replaced Multics (https://en.wikipedia.org/wiki/Multics) as "that other system" that appears frequently in design discussion but nobody's ever actually seen or touched.