I really like the ATP member map.
If people list a recognizable address in their public profile, Dave set up a zoom-able map application that will show who lives approximately where.
http://garden.org/users/member...
For example, near Kingwood TX, I see Moodine, rucky and MGLane. If you wanted to set up a plant swap, you might Treemail them and the 20 or so other members near Houston.
Edited to add:
I see that Dave has added a "Search by location" box to the Memberlist page.
