OpenSolaris is a Sun Microsystems-backed open-source project based on the Solaris Unix distribution. And while Sun is tad late to the party, they still plan to capitalize on the community efforts they have been able to grow so far. OpenSolaris 2008.05 is now available for download, pitching itself as a free solution aimed at developers and system administrators.
Formerly known as "Project Indiana", OpenSolaris also gives the option of a LiveCD that will let you get a taste of the operating system without the need of a full local install. According to Wikipedia, Sun has about 60,000 community members registered on its OpenSolaris site, of which 2,000 are paid for their contributions.
The OpenSolaris community prides itself on stability and innovative features - many of which are shared with the proprietary Solaris - such as using ZFS as the default file system, DTrace, a comprehensive dynamic tracing framework, and a new Image Packaging System (IPS).