I am interested by Literate Programming, in which both human-readable discussion and compiler-readable code are combined. Not only does such a system allow a document to be unit-tested, it generally allows code samples to be more expressive, because you can re-order code sections to fit the discussion. I’ve written a literate programming system for Word and .NET, but this is even cooler: a Wiki whose markup supports literate programming. Every page has a “download code” button that generates both a listing and even a .ZIP file!