Sure, every member of the Fortune 100 is a VMware customer, even rival . But VMware is no longer the only vendor at the top of the x86 virtualization market, according to .

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The prestigious ranking, , places Microsoft and Citrix in the Leaders Quadrant along with VMware. Last year, VMware was listed as the only leader, with its rivals in lower tiers.

Gartner still lists VMware first among the three leaders, with Microsoft second and Citrix third. But given than Microsoft's Hyper-V and Citrix's XenServer are cheaper -- and perhaps nearly as good as VMware -- customers can justifiably seek alternatives. Oracle, Parallels and Red Hat are called niche players in the Gartner ranking.

"Citrix and Microsoft have joined VMware in the Leaders Quadrant by increasing vision and execution respectively," Gartner analyst Thomas Bittman and colleagues write. "Although market share leader VMware continues to set the standard in products and the pace in terms of strategy, Microsoft has increased its market share (especially among midmarket customers new to virtualization), and Citrix is leveraging its strengths and its free XenServer offering to expand its server virtualization share."