A number of new packages are now in the contrib repository including cluster manager, courier-imap, maildrop and piranha - amongst others - thanks to bilbo for these. (we are still waiting for some of the src.rpms)
There are also kernels for i586 in the contrib section.
contrib packages are supplied by third parties, not (necessarily) tested by CentOS developers and not GPG signed by Centos.
contrib is available at http://mirror.centos.org/3.1/contrib and should be on all mirrors.
There is also a 3rd party yum repository for CentOS-3 at http://people.best-off.org/~rhel-devel/centos-3/ with lots of perl nodules amongst other stuff.
The link above provides details of how to configure yum.conf to use the repo.
Dag Wieers also provides a repository of rpms for Rhel3 that will (mostly) work with CentOS - http://dag.wieers.com/home-made/ If you do come across any there that dont work properly then please let either Dag or us know.
You can add Dag's repository to your yum.conf by adding :-
[dag] name=Dag RPM Repository for Red Hat Enterprise Linux 3 baseurl=http://apt.sw.be/redhat/el3/en/$basearch/dag
We welcome 3rd party repositories as being a good way of finding additional software for CentOS, however the CentOS developers do not test the packages and cannot provide support for them.
We will list 3rd party repositories on the CentOS website and also mirror them where practical/legal .
Lance