[b3lvedere]

Depends on what you want to do with it.

Basically anything else that operates on their hardware infrastructure is ok. Though it may be very challenging to get a useful supercomputer on very old operating systems.

I'm guessing one of the major factors to decide for Linux is this:

"The licensing cost of a custom, self-supported Linux distribution is the same, whether you're using 20 nodes or 20-million nodes."