How to list dependent packages (reverse dependencies)?

apt-cache rdepends packagename should do what you want

aptitude has a fairly nice way of handling this:

$ aptitude why bash
i   foomatic-filters PreDepends bash (>= 2.05)

By default, it only lists the "most installed, strongest, tightest, shortest" reason, but you can use aptitude -v why to make it output everything it finds.

apt-cache showpkg <pkgname> 

Example:

apt-cache showpkg lightdm