Rap Genius lets you annotate any documents (obviously, the initial focus is on rap music, but it has plenty of other political & historical docs) in a cool way to explain or further elaborate passages. Go to the Gettysburg Address to see what I mean — they basically replace links with “pop-ups.”
With the language on so much rap music, though, it’ll never make it past school content filters. I assume, though, that other sites will start picking up this idea….
You can read more about it at TechCrunch.