I'm often not much of a GUI tool guy, but Observer is really cool - the GUI really does give you a nice view of how the supervision tree of a more complex Erlang system is put together.
Yep. Not only that, but I find it to just be insanely fun to go double-clicking with wanton abandon throughout Observer, digging in to everything just because I can. That it's all there, with no setup and effort required, makes it feel almost magical.