When server virtualization comes up in conversation, it's easy focus on the "big three," VMware ESX, Microsoft Hyper-V, and Citrix Xen, because of their market share and pervasive marketing. There are other players entering the fray, however. For this segment of our Rolling Review, we brought a couple of less-well-known entities--Virtual Iron Extended Enterprise Edition 4.5 and Parallels Server for Mac 3.0--into our virtualization test lab.

These two smaller players show there's more than one way to run virtualization hosts: Virtual Iron ably jumps through some of the same hoops as XenServer and Hyper-V, and we believe it could challenge VMware ESX in larger enterprises, if the company wanted to. Although Parallels Server wasn't up to our full gamut of tests, being a slightly different beast, it does serve its niche well, running Windows, Linux, and Mac OS X Server instances on Apple hardware.

In terms of performance, every server virtualization product we've tested so far in this Rolling Review has done more than live up to the vendors' claims, showing rapid evolution as the market has heated up. Virtual Iron's and Parallels Server's application performance also pleasantly surprised us.

Like the Xen-based virtualization system from Citrix, Virtual Iron has revved up its performance over the last few years while dramatically easing installation and administration tasks. Parallels leverages its "fat OS" experience with Parallels Desktop to deliver solid performance from its hypervisor, despite the underlying requirement of a full-load Leopard 10.5 OS chewing up system resources and routing I/O.

Both platforms are aimed squarely at the small- to midsize-business sector. Virtual Iron devotes most of its marketing to this segment, although we could see Virtual Iron 4.3's Xen-based hypervisor, centralized management, and per-socket pricing model playing well in larger companies, too.

Parallels Server isn't as mature or feature-rich as Virtual Iron. Its limited storage options, simplified networking capabilities, and lack of migration tools mean Parallels Server likely will be crossed off most potential customers' short lists, unless they're Mac shops. VMware's Fusion is tinkering with Mac OS guest support, but Parallels Server currently is the only solution on the market that lets you virtualize Apple Server instances with Apple's blessing.