Debfoster is a wrapper program for apt and dpkg. When first run, it will ask you which of the installed packages you want to keep installed.

After that, it maintains a list of packages that you want to have installed on your system. It uses this list to detect packages that have been installed only because other packages depended on them. If one of these dependencies changes, debfoster will take notice, and ask if you want to remove the old package.

This helps you to maintain a clean Debian install, without old (mainly library) packages lying around that aren't used any more.