Originally it was the case that Unity could not run in tools like Virtualbox.
How can I now do so?
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Unity has 3D requirements above and beyond Compiz itself.
On top of the 3D required by compiz, which is supported by Virtualbox's 3D support, Unity also requires the OpenGL Frame buffer object extension, which is not supported by Virtualbox's 3D drivers. Wikipedia has a good article on FBO too.)
Bug report: http://www.virtualbox.org/ticket/7835
However this appears to be fixed in the new release of Virtualbox
There is also a ppa for Unity in 2d. It's how I set up my own VM:
Since this question asks about tools like VirtualBox, I can confirm that Unity runs on VMware Player 4.0 and beyond.