It seems to me that most attempts to marry writing with interactivity are usually clumsy and/or facile, but this example from Black Warrior Review is pretty successful.  “The Tetris Effect” is an essay equally about Tetris and how we try to organize and compartmentalize our lives.  You play the game as usual, and each time you complete a line, you’re presented the next paragraph of the essay.  A unique–and compelling–marriage.