The closest open-source example I can think of is --verbose mode on many UNIX utilities, though that wasn't really what I had in mind. Developer Tools in Chrome is another example of the principle - think of how much easier it is to debug Chrome than IE6 - but again, not quite all the way. Imagine using Developer Tools to debug all of Chrome, eg. Webkit rendering (overlaid on the rendered page itself), resource parsing & scheduling, JS JIT compilation, etc, and not just the pages that execute within it. (Maybe this is possible, I'm not familiar enough with the Chrome project to know all the debug flags available.)