Lambda the Ultimate

Pivotal is a document-centered presentation of Haskell. It aims to provide a user-friendly environment in which Haskell documents can be interactively developed and manipulated. A Haskell document is essentially an ordinary Haskell module that may also include free-standing Haskell expressions. When a document is displayed, the values of any such expressions are evaluated and displayed in-place.

Think of a document containing code, comments, and examples, all presented in an aesthetically pleasing manner. The document is live, so you can make changes and see them immediately. This reminds me somewhat of the Lab system in J: interactive tutorials that allow the user to experiment after each new point is presented.