A Brief History of Everything^WUNIX¶
In the Second Age of Computing, the UNIX operating system emerged, in pieces, from the minds of Ken Thompson, Dennis Ritchie, Doug McIllroy and bright sparks from AT&Ts Bell Labs.
From there, the river diverges almost as much as our metaphors. However, the forks we care about start with BSD and System V UNIX R4, which begat SunOS and Solaris. Other documents detail this history in greater depth (and to a large degree they do not necessarily concern us.)
In 2005 Sun Microsystems released Solaris 10 which contains several truly innovative features we continue to rely on to this day. Just as importantly, however, Sun also open-sourced the operating system as OpenSolaris. While there were a few governance missteps along the way, it was a bold move.
If you're interested in the history of OpenSolaris, Jim Grisanzio has written an in-depth piece on it.
In 2010 Oracle acquired Sun, and silently closed the gate. The source code was no longer updated, communications to the open source community stopped. OpenSolaris was effectively dead.
From this, however, project founder Garrett D'Amore took the last drop of the gate and announced illumos in mid-2010.
For a longer, more flavorful description of Solaris and illumos history, Bryan Cantrill's Fork Yeah! The Rise and Development of illumos talk from USENIX LISA 2011 should be considered required viewing. (Note: Potentially not suitable for minors.)
Since 2010, a number of companies have built their businesses around illumos, or are running on one of the distributions built from it. For a very incomplete list, please see who is using illumos.