Plan 9 ... Inferno
I stopped being lazy and checked the wiki. Inferno was created by Bell Labs, and "based on" the experience gained from the Plan 9 development. Then after three years Inferno was sold to VitNuova, who themselves sold it for awhile as a commercial product. Some years later it was released under the GPL, but the last update of the OS (according to Wikipedia) was over four years ago. Inferno seems to be going the way of the original Plan 9.
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