A terrible OS for the Minecraft mod: Open Computers.
Maybe it won't be terrible one day.
It is currently in version Alpha 0004
To install, copy the contents of the source directory onto a disk so that init.lua is in the root directory of the disk.
Not much currently
It't slowly getting closer to something that actually resembles an OS, that's all I can say.
It is now atleast somewhat usable, if you are heavily masochistic.