Red Hat
The vendor says:
Red Hat Enterprise Linux is the leading open source data-center operating system , sold on a per system annual subscription basis that provides continuous value.
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Red Hat Enterprise Linux is the leading platform for open source computing. It is sold on a per system annual subscription basis that provides continuous value (updates, upgrades, support, maintenance, certification ecosystem, etc.). It is appropriate for all commercial deployments – SMB to Enterprise. Red Hat Enterprise Linux couples the innovation of open source technology and the stability of a true enterprise-class platform.
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