とあるOS (ToAruOS) is a hobby kernel and supporting userspace, built mostly from scratch, in development since December of 2010.
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.
とあるOS is open source, released under the NCSA/University of Illinois license. Sources are available on github.
A Live CD build of とあるOS is available here.