Had to get collectd going on a CentOS 5.8 environment today. The EPEL repo was needed to get the packages.
Here’s how to add the EPEL repo to CentOS 5.x
cd /tmp
wget http://dl.fedoraproject.org/pub/epel/5/x86_64/epel-release-5-4.noarch.rpm
rpm -Uvh epel-release-5-4.noarch.rpm