OpenSolaris is an open source project created by Sun Microsystems to build a developer community around Solaris Operating System technology. It is aimed at developers, system administrators and users who want to develop and improve operating systems. As of June 2007, more than 60,000 community members are registered on OpenSolaris.org. An active OpenSolaris User Group community is now growing worldwide, and dozens of OpenSolaris technology communities and projects are being opened on opensolaris.org.
OpenSolaris is derived from the Unix System V Release 4 codebase, and has significant modifications made by Sun since it bought the rights to the codebase in 1994. It is the only open source System V derivative available. Open sourced components are snapshots of the latest Solaris release under development.

History
Sun has released most of the Solaris source code under the Common Development and Distribution License (CDDL), which is based on the Mozilla Public License (MPL) version 1.1. The CDDL was approved as an open source license by the Open Source Initiative (OSI) in January 2005 and is a free software license according to the FSF's definition (see here). Files licensed under the CDDL can be combined with files licensed under other licenses, whether open source or proprietary

License
For the release of OpenSolaris, Sun used a licence which is not fully compatible with the most commonly used free software licence, the GNU General Public Licence (the GPL). Sun's licence, the Common Development and Distribution License is a free software licence, approved by the Open Source Initiative. Due to restrictions in the GPL, a work under CDDL cannot legally use sources from a work under GPL.
The CDDL requires that all attribution notices be maintained, the GPL requires similar attributions.
But on the other hand, files licensed under Sun's CDDL can be combined with files licensed under other licenses, whether open source or proprietary.

GPL compatibility issues

Belenix, LiveCD [2]
marTux, LiveCD/DVD [3], first distribution for SPARC
Nexenta OS, Ubuntu-based GNU/Solaris; LiveCD, VMware image, InstallCD
Polaris, PowerPC port [4]
SchilliX, LiveCD