MkLinux is a port of Linux to various machines done by Apple Research and the Open Software Foundation's research arm.
It is unique in that unlike almost all other Linux ports it is not the Linux kernel running on native hardware, rather, drivers have been developed for the Linux kernel that allow the kernel to run as a task in the MachMicrokernel.
It seems to be completely unsupported by the Linux community and only finds a home on PowerMac's that can't run regular Linux/PPC (I have such a Mac and it is running MkLinux which seems okay).
But running an operating system as a task under another is pretty interesting. I have worked on a product that ran SCO Xenix as a task on top of a real-time operating system which was also a cool trick.
It's also much slower than other Unixes; and not very actively developed anymore. I believe they intended to throw the whole thing at Darwin once. I never found out what became of this.