Something like Google Docs comments hanging off the side of the document? It fits less for the Rap Genius example since the annotations explain the line, and i

Something like Google Docs comments hanging off the side of the document? It fits less for the Rap Genius example since the annotations explain the line, and in code if the line needs explaining then it should have a comment within the code itself. I like the idea for discussions, but does the discussion stay there? Would an individual line get bogged down in "stupid questions" that someone has to trawl through to find what they want? If it's on by default, would it just be annoying to those trying to read the code, and if it's off how many people would go out of their way to turn it on? It's certainly interesting, anyway.