Quoting from NetworkWorld

Market-starter VMware may be still a giant step ahead of the competition, but other vendors, including Microsoft, are working hard at catching up. A look at VMware's competitors:

Microsoft: Currently offers Virtual Server 2005 for free, but plans to ship a hypervisor in 2007 or 2008 that will let multiple operating systems run on a single physical server. Advanced virtualization management tools also are expected around that time.
 
Novell: Integrated the rapidly maturing Xen open source virtualization technology into its July release of SUSE Enterprise Linux 10.

Parallels: The desktop virtualization company plans to begin beta-testing a server virtualization product in the fourth quarter of this year.

Red Hat: Plans to add support for Xen in Red Hat Enterprise Linux 5, due in December.

Sun: Sun's Solaris containers let multiple, isolated applications operate on top of a single instance of Solaris. Sun also will support Xen in a Solaris 10 update expected by year-end.

SWsoft: Its Virtuozzo software virtualizes above the operating system, so multiple instances of an operating system run on top of a single installed version.

Virtual Iron: Xen will be the basis of this virtualization specialist's technology, which lets customers pool virtual resources and tap into them as applications demand.

XenSource: The firm that leads the Xen development project offers XenEnterprise, a packaged, supported version of Xen.

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