We have added several packages to Scientific Linux 4.3 that were not in the Scientific Linux 4.0, 4.1, 4.2, 4.3, or 4.4 releases.

Yum Plugins
yum-downloadonly
yum-fastestmirror
Summary : Yum plugin's expand the functionality of yum. downloadonly allows machines to just download the requested packages without installing them. fastestmirror finds the fastest mirror for your machine, potentially making udates faster.
Added to allow more flexibilty when doing updates. These are not installed by default.