Cent OS

CentOS is the Community ENTerprise operating system.
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CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork.)

The “prominent North American Enterprise Linux vendor” is, of course, Red Hat. Cent is compatible the RHEL. If you need an RPM for some software you’d like to run, grab the “el4” package for Cent4 and all will be well. Probably.