I installed Centos 6.2 and tried to install some packages such as ipython or R but they were not available like many other useful packages that are available by default in other distribution such as Fedora.
The solution is to go to this website:
Go to section “how can I use these packages” and click on epel-release-6.5.noarch.rpm for your system (either 86 or x64)
you should be prompted for a password to install the RPM on your system. Once installed, you should be able to install the missing packages.