Find FreeBSD ports that depend on another port

The current accepted answer is out of date. FreeBSD has replaced its legacy packaging tools with pkgng.

Here's the new way to query the reverse-dependencies of a package:

pkg info -r <pkg name>

Example:

root@server:/usr/ports # pkg info -r python27
python27-2.7.6_4:
        python2-2_2
        talloc-2.0.8
        tevent-0.9.18
        tdb-1.2.12,1
        ldb-1.1.16
        glib-2.36.3_2
        xcb-proto-1.9
        gobject-introspection-1.36.0_2
        samba4-4.0.13

pkg_info answers questions like this. With the -R option it expects a name of an installed port and will display all ports that depend on that port:

pkg_info -R libXfont-1.4.3,1

You can use wildcards to avoid specifying the name with the version number:

pkg_info -R libXfont-\*

Note that this does not work recursively, and thus you need to do pkg_info -R again for each port in the resulting list until you get to the bottom of things.

Note that on servers it is often a good idea to put the following in /etc/make.conf:

WITHOUT_X11=yes

That will make most (all?) ports to skip dependencies to any X11 related stuff.