An on-line editable (wiki-like) story directed-graph mixed with a state machine.
So basically you get a window with the main vertex, some text based on your current state, and links to other vertices describing what action/choice clicking on that link will do/imply/&c.
Embedded in the text is a simple CPP-like programming language that changes state as you land on vertices and go about your day.
TiddlyWiki-like navigation/editing interface and moderator pages with lists of recent changes, abandoned nodes, &c. (perhaps also "this subtree is X deep and Y wide" after the links, though that would require something clever with back-references (i.e. the graph-part))
Inspired by
http://www.asstr.org/~Arthur_Saxon/Your_Wild_Day_js.html