I think the relationship with Vagrant has helped Oracle's VirtualBox significantly in popularity. Yes, Vagrant works with other virtualization platforms, but since it so seamlessly integrates with VirtualBox, I think it gets quite a boost. Virtualization is such an efficient and reliable way to implement systems, bare-metal solutions are almost a thing of the past!
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Oracle VM VirtualBox 33.4%
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VMware 22.3%
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KVM 21.1%
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XEN 5.7%
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QEMU 5.3%
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OpenStack 4.9%
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Other 4.2%*
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OpenVZ 1.7%
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Linux-VServer 1.3%
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Symantec Workspace Virtualization .1%
*Under "Other", the most write-ins went to Docker, ProxMox and LXC, in that order.