OpenSolaris

OpenSolaris: What is it exactly?

OpenSolaris also commonly referred to as osol, is a completely free and open source version of the Sun Microsystems Solaris operating system, licensed under the CDDL. It is also the code base for the up and coming Solaris 11.

It is most commonly distributed as a live CD, that can be installed to your hard disk, in the same way that many modern Linux distros can. It makes use of the Gnome desktop, although others are available. If you want to take it for a spin, and get a feel for what it's all about you can download it here.
http://www.opensolaris.org
Click on the Get It! Link, and you will be directed to the download page.

OpenSolaris: A platform for VistA?

Utilizing a virtualization technology called zones (or containers which is slightly different), the Linux version of GT.M (and thus VistA) run un-modified. You still have a great deal of the functionality that exists in Linux, in addition to technologies only found in Solaris operating systems. A couple of the most popular and significant being ZFS, and Crossbow.