TiddlyWiki is not new; it started its way in 2004, but development is still going strong today, eight years later. It is one of the simplest Wiki solutions available: Nothing to install, no server software to configure, and no special permissions to set. You just download a ZIP archive containing two files, open "empty.html" in your favorite browser, and off you go. The second file in the ZIP is called TiddlySaver.jar, and is a tiny (5KB) Java applet used for saving your changes to disk. It is only needed on some browsers, such as Google Chrome.
Since TiddlyWiki is built as a single document, it doesn't use pages. Instead, individual topics are called "tiddlers." You can have as many tiddlers as you want and even show them all on the screen at the same time. When you're done with a tiddler, you can click its "close" link and it will be hidden away until the next time you need it. If you want to focus on just one particular tiddler, you can click "close others" to close all other tiddlers cluttering the screen.
You can edit tiddlers in place: Just click the edit button, and change anything you want. You can format text using simple wiki markup: Surround a word with double slashes to make it italic or with two sets of single quotes (like ''so'') to make it bold. You can also highlight text, underline it, and use superscript and subscript. Most importantly, you can link tiddlers using CamelCase or [[double brackets]].
Wikis can grow large, even when they are created and maintained by just one user. TiddlyWiki lets you apply tags to each tiddler, so you can see all tiddlers related to "finance" (for example) with just one click. It also has a live search feature, which brings up relevant tiddlers before you even finish typing.
While TiddlyWiki's default feature set is robust, you can tweak things even further with plugins and themes. Plugins can automatically add tables of contents to individual tiddlers (if you like keeping long tiddlers), create collapsible sections within tiddlers, and more. You can find these plugins on community website TiddlyVault, and there's a showcase of themes on TiddlyThemes.com.