There is the proprietary closed Solaris kernel and the closed Solaris distro: Solaris 11 by Oracle. It is free for personal use (testing and development), but not free for production.

The Oracle Solaris 11 source code has been closed, and then the open source community forked the Solaris 11 source code. Illumos is the forked OpenSolaris kernel by the open source community. There are several OpenSolaris distros that uses the OpenSolaris kernel: Illumian, Nexenta, OpenIndiana, etc.

Solaris can not run Linux program anymore. Earlier you could, in a Zone (similar to FreeBSD jail). But the OpenSolaris distros are considering to add the Zone functionality again (it is called BrandZ). You can run Linux software in the open sourced VirtualBox program.

You can set up a network in Solaris/OpenSolaris with the extremely powerful functionality called Crossbow. Linux has cloned this functionality and called it Open vSwitch.

There are very powerful communication abilities in Solaris/OpenSolaris. I heard that much of the functionality, for instance NFS(?) where created by Sun in Solaris. Many of the Unix gurus, worked at Sun at one time. Solaris has been very innovative and many OSes has cloned Solaris functionality: Linux has cloned BTRFS (Solaris ZFS), SystemTap/KTrace (DTrace), Open vSwitch (CrossBow), (Zones), etc etc etc.

Linux is a clone. Solaris is the original, which Linux tries to clone.

Thanks very much.

OpenSolaris is discontinue !!!!

Some parts of OpenSolaris are actually still free and open source software but the Solaris 11 ON (kernel and core utilities) is no more. Its Illumos fork is still free and alive.

openindiana

OpenSolaris is forked. The new forked community driven version is called OpenIndiana. Another fork is called Illumian.
http://openindiana.org/
http://illumian.org/