EpicDoc is an OpenSource documentation tool based on Ext JS.
It's an enhanced and standalone wiki system based on web technologies without having the annoying drawback of being tied to a web server architecture. But it still leaves you the opportunity to bring the content to web very easily.
Just take a look here: EpicDoc documentation
This project is in the initial phase and pretty much alpha right now.
If you like the idea, you're very welcome to join. Help is very appreciated ;-)
As long as I'm the only person the progress will be very slow (I'm developing in my spare time) and this tool will be developed the way it fits my needs the most :-P