There are currently two primary open source projects that have been useful at the company for which I work and locally.
The first, gkrellm, is a graphical system monitor that works in a networked environment. I have some patches for this software that make direct use of iptables information to do packet accounting. This requires that the software run as root which may not be desired for some users. I don't run this software much any more, so the patches aren't kept absolutely current to the latest gkrellm release upstream. In the past, the patches were able to applied to Bill Wilson's latest source trees. Your mileage may vary.
The second is the yum-pull repository manager. This was written by William Stearns. It was frequently a release or so behind for the software that we needed where I work, so I modified it and cleaned it up, removing the repositories which we would never use and generalizing the release management. It isn't as flexible as his is operationally, but it is much easier to update as new releases of software are released. We kept the Fedora, CentOS, RHEL, and kde-redhat repositories, and added the fusion repositories. We also added quite a few new option switches to make our life easier.
Along with this script is a checkmirror script written in perl that I added to try to prevent downloading RPMs that were out of date with respect to the RPMs already available in the local repository. I've also added some patches to try to not remove kmod and xorg drivers for old kernels to make it easier to revert back a kernel level if something goes wrong with the most recently produced kernel. The verifymirror script is a helper script in case your local mirror has hardware problems that takes out part of your repository. The managemirror script speeds up the deletion process for old RPMs on your mirror.
Many thanks to the original authors of yum-pull and GKrellM and for releasing them under the GPL.