Tiddly Wiki

http://www.tiddlywiki.com/ is a free product with these salient features:

1. All data and functionality is stored in a single HTML file, so you can run it in your browser without needing to install anything. You can take the file with you on a USB drive.

2. The form of the software is a series of user-created "tiddlers", which are akin to notes, entries, or articles. The brilliant thing about it is that you can associate tiddlers with each other via hyperlinks, either as embedded in the test or as menus and tags.