It started when I wanted to create a User Friendly plugin for iComic.
iComic has an "open architecture, supporting plugins written in perl." Neat. So I found one of my plugins in ~/Library/iComic/Plugins and dragged it to TextEdit. No dice, didn't open. Odd, I thought, the plugin should just be perl. So I dragged it to BBEdit, which launched a type of window I hadn't yet seen - the Disk Browser. Come to find out, the plugins are more like application packages.
Disk Browser mode is a two-pane window with file navigation on the top and text editing on the bottom. Looks great for switching back and forth between resources and property lists. Neat.