What is the equivalent of packages.debian.org for CentOS?

There's no such official site, however, you can view the repositories (found in /var/cache/yum/base/mirrorlist.txt) in a webbrowser.

For example, http://mirror.stanford.edu/yum/pub/centos/5.5/os/x86_64/CentOS/ lists all 64bit packages of CentOS 5.5 hosted by Stanford University.

There's a repository called EPEL, maintained by the community behind Fedora, with some extra packages.
They use a web tool called repoview to show their packages. It's more pleasant than just staring at a list of package names, and provides descriptions for each package.

Check it out at http://download.fedora.redhat.com/pub/epel/5/x86_64/repoview/ .

You can look for RPM packages in http://rpmfind.net it's a very good complete repository

check out http://mirror.centos.org/centos/5/os/i386/CentOS/ and http://mirror.centos.org/centos/6/os/x86_64/Packages/ you can find all the centos packages of different architectures in these url's.