Okay, I heard about many unusual OSes, but these...

[asian1]

Hello
Plan9 is a Unix look-alike (but not UNIX) that run on 4 platforms including PowerPC. The OS is created by Bell Labs (the same research institution that created UNIX).
The succesor of Plan9 is Inferno, with Virtual Machines, similar to Java and AmigaDE. Unfortunately the top executives at AT&T and Lucent does not understand the potential of Plan9 and Inferno. There is a portability tools and library on BSD for porting Plan9 applications.
I used Plan9 around 1993.

FAQ

LynxOS is a Real Time OS based on Unix. There is a portability library on LynxOS that enable Linux application to run on LynxOS.

Linux ABI on LynxOS