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Some form of Linux, then compile as needed I imagine.

The page above lists RHEL, SLES (the commercial variant of OpenSUSE), Ubuntu in the context of servers.

Debian as well. IIRC there is a FreeBSD porting effort which hasn't made that much progress as yet.

Link to the Debian port:

More than 96% of Debian is compiled for it and hopefully works ;)

I've personally run a KDE environment on ppc64el Debian, and it pretty much works straight out of the box :D