In 2005, Sun released the source code for their Solaris operating system under the terms of the open-source CDDL license. This project was named "OpenSolaris", and a new website and community appeared, at http://www.opensolaris.org/.
OpenSolaris is not itself an operating system - rather, a community built around the open-source parts of the operating system. So, you cannot download and install OpenSolaris; instead, in a similar way to GNU/Linux distributions based on the Linux kernel, there are several OpenSolaris-based operating systems. The most established of these is Solaris itself, which is available in binary form as Solaris Express.
This question was asked on the wiki: OpenSolaris.