Until recently, there have not been may seriously viable free/libre options for virtualization (running one operating system in a guest virtual machine on some other host system is the common setup/pattern for this). There was Bochs, which was an emulator, not a virtualization environment, and was thus highly slow. There was qemu, which (when coupled with kqemu) had reasonable speed, but still had some lag. There’s Xen, which is a heavyweight solution that seems to be primarily applicable to servers and also seems (unless coupled with Red Hat’s tools) to be rather poor for quick-and-easy VM creation and destruction.

There’s always the proprietary options such as VMware. This is feasible for the hobbyist now that VMware has made their server product free/gratis (I use it to run my mail/web server VM on our desktop), but it’s still proprietary. I’m not the biggest fan of proprietary things in my kernel (although VMware’s stuff does seem to be solid).

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