TiddlyWiki

TiddlyWiki is a unique, non-linear personal wiki contained in a single HTML file. It consists of 'tiddlers' (micro-content notes) that can be linked and tagged. It's incredibly customizable through plugins and theming. It's ideal for tinkerers, researchers, and anyone wanting a portable, highly adaptable wiki system that runs in a browser.

It's free and open-source. It scores for its legendary flexibility and portability, though it has a steeper learning curve than modern apps.