The mighty Sun Solaris operative system has been for years a reference when it comes to speed and robustness, and has deeply influcenced the evolution of modern Unix systems. On 14 June 2005, Sun decided to open the source code, which you can find and download at http://www.opensolaris.org/
Links:
[http://en.wikipedia.org/wiki/Solaris_Operating_Environment Solaris wikipedia page]
[http://developers.sun.com/prodtech/solaris/reference/techart/ Technical Articles and Tips]