Tiddly Wiki Classic

TiddlyWiki -- one-file Wiki implemented in JavaScript. Strong community; many different (and interesting) adaptations at http://tiddlywiki.com/

Classic -- The original. More mature, has more useful plugins, and requires an additional JSON file stored in the same folder of your tiddlywiki when using Chrome or IE.
TiddlyWiki5 -- TiddlyWiki rewritten from the ground up based on HTML5 standards in order to to be as future-proof as possible. Currently in beta, lacking many features of the original (using plugins)(3/4/2014).