I suggest "dpkg" and its sisters as major projects.
I think that the most important project under development in the open source world is the Debian package management suite (dpkg, apt, gnome-apt, dselect, deity,
that script that makes graphs of dependencies, debhelper & company, lintian, cruft, popularity-contest, that test-your-debian-packages package, that script that downloads the source and compiles up an optimized Debian package for you, etc.).
Likewise, its poorer cousins rpm, rpmfind, rpmfind.net,