How can Growl trigger an shell script or AppleScript (NOT vice versa!)

When a Growl notification gets issued I want to trigger an AppleScript. I had a look at the "Actions" in the Growl preferences, and you can trigger things like an email or a text-to-speech call, but not an arbitrate shell script or AppleScript. Any idea how to do this? Does a Growl plugin for this exists? I searched the net for it, but could only find solutions for the other way around (i.e. issuing a notification from a script). Many thanks in advance!