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I'm currently working with an HPC with 65000+ nodes (it's in the TOP 500, well TOP 100 actually). It's running RHEL 5. I have 12 years experience in IT and all I have seen is RHEL, RHEL, RHEL and CentOS (same as RHEL). I've seen some Oracle Linux (still same as RHEL) here and there. Some small businesses use Debian (mainly for web servers) and I've seen some Suse in large companies, which they are replacing with RHEL.
Don't forget Scientific Linux (~CERN, so also some HPC here and there I imagine).
Still same as RHEL...
I've never encountered any one of those in my carrier though but I imagine it exists.