wajig
The package management of Debian and its forks is arguably one of the best ever created. Its user interface evolved from the primitive dselect, over the ubiquitous apt-get, to aptitude, the current standard (of which synaptic and adept are graphical implementations). Aptitude resolves dependencies very reliably and is, for example, also able to clean the system from orphaned packages.
Aptitude, however, is not sufficient for the entire range of tasks for package management, but requires additional helper-tools such as apt-cache and dpkg. For this reason, I use wajig, a unifying python script which simplifies package management in Debian (or Ubuntu) enourmously. Wajig combines the abilities of apt-get, apt-cache, and dpkg under a single consistent interface, and it is much easier to use than aptitude. I wonder why so few people know and use wajig...