User-Mode Linux is a safe, secure way of running Linux versions and Linux processes. Run buggy software, experiment with new Linux kernels or distributions, and poke around in the internals of Linux, all without risking your main Linux setup.
User-Mode Linux gives you a virtual machine that may have more hardware and software virtual resources than your actual, physical computer. Disk storage for the virtual machine is entirely contained inside a single file on your physical machine. You can assign your virtual machine only the hardware access you want it to have. With properly limited access, nothing you do on the virtual machine can change or damage your real computer, or its software.
This would be useful as a secure sandbox for a ReflectiveWiki.
It is useful for virtual hosting multiple wikis or even a WikiHive of related wikis, for about the same price as shared hosting but with more disk space.
This brilliant idea started as a separate patch, but has gone into the 2.5 development kernel, so it will be available to everybody when the 2.6 kernel is released.
UML commercial hosting providers