ToAruOS (とあるOS)

ToaruOS is a hobby operating system, including a kernel and userspace, built from scratch since December of 2010.

Quake Quake

Gears Gears

It was originally developed at the University of Illinois at Urbana-Champaign. For a period of time, it was the development focus of the university's SIGOps chapter.

ToaruOS is open source, released under the NCSA/University of Illinois license. Sources are available on gitlab.