Parallels has announced the first beta release of Parallels Server, a new virtualization program that, like Parallels Desktop, lets one operating system run as a virtual machine inside another. But Parallels Server introduces some significant new capabilities, not the least of which is support for running Leopard Server as a guest operating system. Thanks to Apple's recently updated licensing terms (see "Apple to Allow Virtualization of Leopard," 2007-10-31), owners of Leopard Server can run it as a virtual machine - and even run multiple copies of it on a single computer - as long as each copy is purchased and licensed individually and the host computer is made by Apple. (Parallels Server itself runs on Mac OS X, Windows, or Linux.)
The option to run two or more copies of Leopard Server (along with other operating systems, such as Windows Server and Linux) on, say, one of the spiffy new eight-core Xserves (see "New Xserve Goes Eight-Core Too," 2008-01-08) could prove to be very interesting to sites needing to get the most flexibility out of a limited number of machines.
In addition to guest support for Leopard Server, Parallels Server finally offers (limited) support for multiple processors or cores in guest machines, a capability the company says will migrate to Parallels Desktop in the future. Among the other new features is the option to install and run guest operating systems using a "bare metal" hypervisor that eliminates dependence on the host operating system. Since I haven't seen this capability in action personally yet, I'm having some trouble grasping exactly how it will work, but it certainly sounds interesting.
The beta testing program for Parallels Server is private, meaning that registration is required, though apparently Parallels has opened participation to anyone.