For the last few days I have been experimenting with Apple's Quartz Composer. While this is primarily a motion-design tool (and a very powerful one indeed), it is also an example of a very effective graphical programming environment. Prior to this, I had seen such tools in the Windows environment and was less than impressed, but QC is really amazingly powerful; you can parse structures, use variables, loops, and everything somehow fits together very well. I see uses for this metaphor in the BPEL world, at the very least, but the whole world of distributed computing seems a good fit for it.
By the way, this is the 'code' behind one of the rather phenomenal demos that can be found here.