The fact that it's freely available has made Oracle's VM VirtualBox something of a sleeper hit in virtualisation (even if it doesn't have major market share). The just-released version 4.3 adds support for a number of recent operating system releases, including the just-about-to-launch Windows 8.1.
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The Windows 8.1 support includes ten-point multi-touch, which is useful for tablet devices. Windows Server 2012 R2 and OS X 10.9 have also been added, as well as a virtual USB webcam for use in communications apps. VirtualBox is a free download for Windows, Mac and Linux for individual use.