Once upon a time, Sun Microsystems produced Solaris, one of the rather popular flavours of Unix. They even opened up much of the code for interesting portions of the code base, calling it OpenSolaris. Oracle has been rather less interested in promoting the open bits, so a fork called Illumos was established to ensure that copies of code would continue to be available, and that hopefully development efforts could continue. Additional forks of that have emerged, which is hopefully good news.
Interesting features include:
A high performance filesystem offering many database-like features
A framework allowing one to trace what's going on broadly throughout code of your system, including some hooks that get reasonably deep inside the kernel.
Inspired by BSD Jails, this provides a lightweight form of virtualization
A popular hypervisor
If this was useful, let others know by an Affero rating