Red Hat are the company that have most 'commercialised' Linux, but have given most to the community in the process.
It's a distribution, invented and uses rpm(8).
A good source for up to date RPM files is http://www.freshrpms.net (also has a how-to on using apt with RPM)
They offer a certification (RedHatCertifiedEngineer) and lots and lots of corporate support.
Part of CategoryDistribution
120 pages link to RedHat: