Plan 9 is a research OS produced by Bell Labs. It was open sourced a few years back, and has enjoyed a revival of sorts. Los Alamos National Labs is continuing to favor Plan 9 for their new generation of super computing because its the fastest thing out there. I have downloaded and ran Plan 9 before. In fact the Plan 9 live cd sits here on my desk. Its not an operating system for noobs however, and lacks some graphical refinement. Plan 9 is a very cool and a interesting test drive however. Its definitely worth the price of admission (free) for exploring, and education.