Plan 9 from Bell Labs (the link to THE website)
The last time I gave Plan 9 a try was, when I worked with Derwent in London. That was pretty much in vain then, I just couldn’t really get it running on a PC. But nowadays they provide you with images for virtual machines. I still assume, if Plan 9 had ever gotten to enough speed, there would never have been GNU Hurd and Linux either.
This time I came across Plan 9 in the context of Google’s Go programming language, which they seem to provide for Plan 9.