TiddlyWiki, a rather nifty-looking JavaScript/DHTML-based wiki running entirely in the browser. It doesn't actually have a back-end, so it's read-only unless opened from a local file in Firefox. As used in
Charlie Stross' slightly facetious guide to the Singularity.
Update: And here is a properly AJAX version of TiddlyWiki, which uses a PHP back-end to store entries.