This is fantastic, and revolutionary — redefines what a debugger / programming environment should be. Thanks for making it.

(I had a vague idea to do something similar for a specific program that has a reputation for being hard to debug (TeX); will definitely look to your code for inspiration.)

Have you looked at Python Tutor (http://www.pythontutor.com/)? That's another excellent project that has some overlap — e.g. you may look at its insertion sort example (go to http://www.pythontutor.com/visualize.html click on "Show example code and courses" and click on "Insertion sort"). If you have looked at it and can summarize the differences in approach between your project and that one, it would be highly useful.

Thanks for an inspiring project! This is so obviously the right way to do things that in hindsight it's a shame we're not already using it. :-)