CentOS / RHEL / Scientific Linux 6 Enable & Install EPEL Repo

Extra Packages for Enterprise Linux (or EPEL) is a Fedora Special Interest Group that creates, maintains, and manages a high quality set of additional packages for Enterprise Linux, including, but not limited to, Red Hat Enterprise Linux (RHEL),CentOS and Scientific Linux (SL). How do I enable it under CentOS 6 or RHLE 6 server?

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