It's a Wiki site encapsulated into one HTML file. You view and edit it like a Wiki but save it locally. It's your very own web notebook.
Here at Esendex, we're big fans of Wikis having an internal one which contains a wealth of knowledge on how our company operates. This includes, system documentation, sales and finance processes, system performance records and a whole host more.
What's really neat about Tiddlywiki for me is that I can push this HTML file to my Nokia N95 and it works a treat. I've got access to everything I want to make notes about, wherever I am. All I've done is saved the file to a synchronisation folder on my PC so it's always in sync.
The only thing I can't do currently is save changes made on the phone which I'll investigate further. In the meantime I'm definitely moving from a random collection of notes in Outlook to this far more useful contextual notebook.