RLX is a rogue-like video game engine written in Emacs Lisp that is being ported to Common Lisp.
In addition to the fact that this is a game so it is just plain fun, the author uses emacs as an IDE for creating games using the engine.
What more could you ask for? Read more on about it on his blog.
Michael Weber had the inspiration to imagine how might one visualize Lisp without parentheses in his mwe-color-box.el extension to Emacs.
Below is a screenshot from Michael’s page. More of Michael’s Emacs Hackery is available here.