Wikipedia
OpenSolaris is a discontinued, open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software. After the acquisition of Sun Microsystems in 2010, Oracle decided to discontinue open development of the core software, and replaced the OpenSolaris distribution model with the proprietary Solaris Express.
Prior to Oracle's moving of core development "behind closed doors", a group of former OpenSolaris developers decided to fork the core software under the name OpenIndiana. The OpenIndiana project, a part of the illumos Foundation, aims to continue the development and distribution of the OpenSolaris codebase. Since then many more illumos distributions are available for use, continuing development in open or offering support.
OpenSolaris is a descendant of the UNIX System V Release 4 (SVR4) code base developed by Sun and AT&T in the late 1980s. It is the only version of the System V variant of UNIX available as open source. OpenSolaris was developed as a combination of several software consolidations that were open sourced subsequent to Solaris 10. It includes a variety of free software, including popular desktop and server software. On Friday, August 13, 2010, details started to emerge relating to the discontinuation of the OpenSolaris project and the pending release of a new closed-source, proprietary version of Solaris, Solaris 11.