AMMDI is an open-notebook hypertext writing experiment, authored by Mike Travers aka mtraven. It's a work in progress and some parts are more polished than others. Comments welcome! More.
Extensions: the combo of Lumo and AppleScript opens up a lot of possibilities. Just with the browser, imagine something that tracked your activity for you, learned patters, offered suggestions and/or a better memory than the really bad browser histories.
There are plugins like this, never quite found one I use though.