User-Mode Linux lets you run Linux inside Linux. It 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. 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.
The Yoke driver provides transparant mirroring between disk partitions and other block devices (such as loop and nbd). It differs from the kernel RAID0 facility in that it is a virtual device with no interface of its own. To start mirroring one device to another requires no prior setup, just "yadd /dev/yd0 yet_another_device" and go. The yet_another_device will become another facet of the existing mirror and you can still read and write to it perfectly normally, with the difference that all changes will be seen and reflected in/from all the other mirror facets.
Gentoo Linux is a versatile and fast Linux distribution for x86, PowerPC, Sparc, and Sparc64 that's geared towards Linux power users. Unlike other distros, it has an advanced package management system called Portage. Portage is a true ports system in the tradition of BSD ports, but is Python-based and sports a number of advanced features including dependencies, fine-grained package management, "fake" (OpenBSD-style) installs, path sandboxing, safe unmerging, system profiles, virtual packages, config file management, and more.