How to list installed ports that have no dependant ports

[Adam]

(/usr/ports) - (eskimo at ttypb.jake) - (01:31:37)
-$ pkg_info |wc -l
     488

As you can see, I need to clean up my system quite a bit. I've got a ton
of things installed, most of which were installed as dependencies for
ports that have since been uninstalled.

So, what I'd like to do is list all the ports installed on my box that
have nothing dependant on them. In this way, I could start removing
things that I don't need anymore. 

Is there any clever way to do this? I've toyed around with a few ideas,
but haven't been able to come up with anything that works nicely.

-- 
Adam <blueeskimo at gmx.net>