TiddlywikiThe genius of the Tiddlywiki is its simplicity. The wiki is a self contained single html file that has inside it all the code formatting and data that magically becomes a wiki when viewed with a browser.
Thus the tiddlywiki could run off a webserver like apache or carried around in a laptop or USB drive. You could also have your Tiddlywiki hosted online.
Tiddlywiki has the ability to be customized. Snippets of functionality, referred to as Tiddlers developed by a vibrant community of developers, can be imported into a tiddlywiki as required by the user. The "tiddly tool" include contact/address books, clocks, browser,Mindmap tools, GTD tools and more.
Many customized versions of Tiddlywiki are available, thanks to the efforts of these developers Below are a few useful links from Lifehacker for those who choose to explore this little marvel:
Host your TiddlyWiki at Tiddlyspot
Monkey GTD Tiddlywiki
Adding an addressbook to Tiddlywiki
Tiddlywiki Student Edition
Grab Text for your TiddlyWiki with TiddlySnip