TiddlyWiki

I just have to say that TiddlyWiki is one of the coolest hacks I've seen in a long time. For those of you who don't like to blindly click links, I think this blurb from the website sums it up quite well:

One of the neatest features of TiddlyWiki is that it is entirely self-contained in a single HTML file. It contains the actual hypertext document, and the JavaScript, CascadingStyleSheets and HTML necessary to both view and edit the document. This means that it is trivial to host a TiddlyWiki on a website, or to distribute one by email. And anyone with a reasonably recent web browser will be able to read and edit it.

If you don't know what a Wiki is you may not care, but my amazement at this thing has steadily grown in the few days since I ran across it. The WikiOnAStick idea in particular is my favorite aspect so far.