OpenSolaris is an open source Operating System based on Solaris created by Sun Microsystems. OpenSolaris is a descendent of the UNIX System V Release 4 (SVR4) codebase developed by Sun and AT&T in the late 1980s. It is the only version of System V available as open source. OpenSolaris is developed as a combination of several software consolidations which were open sourced subsequent to Solaris 10. It includes a variety of free software, including popular desktop and server software.
As this article release, the current version of Open Solaris is 2009.06 with many new features, as you can see in their website here.
You can download OpenSolaris 2009.06 here.
And you can see the documentation here.