MkDocs is a fast, simple and downright gorgeous static site generator that's geared towards building project documentation. Documentation source files are written in Markdown, and configured with a single YAML configuration file.
TiddlyWiki is a complete interactive wiki in JavaScript. It can be used as a single HTML file in the browser or as a powerful Node.js application. It is highly customisable: the entire user interface is itself implemented in hackable WikiText.