dbackup is a disk-based client-server backup system for Linux or other UNIX systems. It works on the principal that disks are cheaper and more reliable than tapes.
Windex is a Perl script which prepares an HTML document containing a graphical index for a collection of image files, with each small thumbnail image linked to the corresponding full-size image.
Darkstat is a network traffic analyzer. It's basically a packet sniffer which runs as a background process on a cable/DSL router and gathers all sorts of useless but interesting statistics.
libpng is the official PNG reference library. It supports almost all PNG features, is extensible, and has been extensively tested for over seven years.
Well known chess motor. It does not have any usefull user enviroment, but has XBoard compatible protocol so you need any program like XBoard or SCID to use GNU Chess.