Generally, Freshmeat has most OSS (and much non open-source) software catalogued.

If you use an RPM-based distro, rpmfind.net is good if looking for a specific package (be careful: don't install RPMs that aren't designed for your own distro unless very similar)

The Freshmeat directory is the best place really. http://freshmeat.net/browse/18/?topic_id=18