centos 5,6,7 How can I use these extra packages?

http://fedoraproject.org/wiki/EPEL

How can I use these extra packages?

EPEL has an 'epel-release' package that includes gpg keys for package signing and repository information. Installing this package for your Enterprise Linux version should allow you to use normal tools such as yum to install packages and their dependencies. By default the stable EPEL repo is enabled, there is also a 'epel-testing' repository that contains packages that are not yet deemed stable.

NOTE for RHN users
You need to also enable the 'optional' repository to use EPEL packages as they depend on packages in that repository. This can be done by enabling the RHEL optional subchannel for RHN-Classic. For certificate-based subscriptions see Red Hat Subscription Management Guide

NOTE for CentOS users
You can install EPEL by running yum install epel-release. The package is included in the CentOS Extras repository, enabled by default.

If you are running an EL7 version, please visit here to get the newest 'epel-release' package for EL7: The newest version of 'epel-release' for EL7

If you are running an EL6 version, please visit here to get the newest 'epel-release' package for EL6: The newest version of 'epel-release' for EL6

If you are running an EL5 version, please visit here to get the newest 'epel-release' package for EL5: The newest version of 'epel-release' for EL5

You can verify these packages and their keys from the Fedora project's keys page: https://fedoraproject.org/keys