CentOS

CentOS is a clone of Red Hat's Enterprise Linux (RHEL).

CentOS is based on the same source RPMS, except for modified artwork, trademark, etc. This means near-perfect binary compatibility with RHEL, and it probably means that CentOS will be supported for a very long time because the CentOS developers just need to re-compile Red Hat's source RPM packages. That way, your CentOS installations have a good chance of a long life without need for re-installations.

CentOS doesn't have the same level of support options as RHEL, but is free ("as in beer"). Thus, applications like Oracle (which targets very specific Linux distributions, including RHEL) easily installs on CentOS.