Their is a project called easylife @ http://easylifeproject.org/
It will install, configure and add extra repositories to yum's source list. The site says easylife is for fedora and since scientific is also based on fedora, it can be use as well.
Easylife does not replace the yum package manager. Easylife just adds extra repositories from rpmfusion, livna and others to yum's source list.
The first time you use easylife it will download the plugins/packages you need.
When you want to install software in the future -- just use yum as usual.