TiddlyWiki is a single-file client-side wiki written in JavaScript and HTML, and it's the only one besides Wagn that I've seen do transclusion in a graceful way, with accordion collapsing and double-click-to-edit. When I write project reports I like to write headings and content non-linearly, and move things around as I edit, and collapse sections so that I can see the pattern of my report easily. Wagn lets me do this, and -- for anyone without Ruby on their computer -- Wagn isn't desktop software. TiddlyWiki is. Wagn seems more powerful for large community sites and manipulating structrued data, whereas TW is focused mostly on personal information organization and display (with the RippleRap derivative for team note sharing at conferences without constant wi-fi connectivity).

Another useful feature of TW is "tabbed lists" -- really slick display format: http://tr.im/RpVO

It's used as a CMS, blog, GTD tool, and much more: http://www.tiddlywiki.com/#Examples