Live UI Editing
Some language communities (CommonLisp is probably the largest, most obvious example) are used to incremental development, live updates, online bug-fixing and the many other benefits of a language with a REPL.
Mike Austin is currently working on bringing some of this style of development to the GUI arena with his work on Live UI Editing. Hopefully he'll release a demo in the not too distant future. Thomas Sutton 27 July 2005