A collaborative wiki, with a LISP syntax. And I don't mean the wiki engine is written in LISP. I mean the page content is.
Functional programming seems to have become chic again. Coming from an OO background, I have and little exposure to functional programming, save from some Scheme as an undergrad (you remember that, don't you?). Scala was a nice gateway language for me. I started to see the advantages.
So, if you've ever wanted to write a link that looks like this:
(defun link (url text) (concat "<a href=" url ">" text "</a>")) (link "http://kevinp.me" "Kevin's Page")
LambdaWiki might just be for you. Try it out and check out the code, and let me know what you think!