I'm known as a hater of RPM/Yum based OS, just because i think their lacking of decent package repositories (at least for recent software).
While in Debian/Ubuntu, there are a lot of good repositories with recent packages. I'm not "bleeding edge" admin (for that i could use Gentoo) i just don't like being forced to install things from source or trust any random RPM found on the web.
I know any decent sysadmin should be able to maintain is own repositories, but since I'm mainly a programmer i like to leave this work to a 3rd party.
What are your choices to maintain software/repositories on CentOS/Fedora?
There are also lots of not so active / small repos for some specific applicationss. You could try digging thru opensuse build farms or just google with two keywords like:
repoview "jump to letter"
I haven't used Fedora in quite some time, but I use CentOS a lot, and swear by the RPMForge repositories. The CentOS Wiki has instructions for setting up "repository protections" that will stop RPMForge from replacing core CentOS packages that it has bigger version numbers for. This makes it quite safe to add the RPMForge repo
(I had to insert the link as a comment because new users are not allowed to post links.)
Centos has a few repos of its own; RedHat sponsors Epel which contains a limited, but high quality number of Fedora packages ported to RHEL.
rpmfusion.org is a very good repo for Fedora. RPM Fusion is a merger of Dribble, Freshrpms, and Livna.
I have found Dag Wieers repository useful.
My favorite repo for Fedora was already mentioned (RPM Fusion), so I'll point out that some companies, Adobe and Google for example, are starting to provide their software via yum.
Also, I would say that setting up a repo is simple enough that you might still consider it. Run createrepo against a directory of rpms, expose the directory via http, and you are done.
Tread lightly with Dag repos in CentOS. Normally, they're quite okay. But every so often we've had trouble rolling back when things don't play nice.
That said, it's not often that I need to resort to dag in CentOS.
I generally use or search RPMs or Packages on http://rpm.pbone.net/
It gives a history and details on files and dependencies too.
I'm quite fond of axel thim's atrpms.