First visit CentOS wiki regarding third party repositories as suggested and then read the rest of this post.
I've made
this thread with links to repositories I use to SEARCH for packages. Day to day use came down to enabling centos and extras, and using epel and elrepo. I also put together a small repository with downloaded packages from atrpms-stable, rpmforge and few others.
What ever you do, DO NOT mix atrpms with rpmforge on the daily basis, dependencies are all over the place with that lethal combination.
I am thinking of a branch of CentOS to be used as desktop and having a selection of programs to cover every day usage in that capacity. Point in that direction is plnet-centos-bundle package with dependencies to number of packages missing from original CentOS release. It's in plnet-releases repository, and should be a base for carefully designed set of such packages covering needs of wide variety of CentOS users (with and without proprietary software, etc...).