The Avalon Documentation Engine (also known as ADE) is a free and open-source piece of software, that lets site administrators manage content in a simple format.
The Avalon Documentation Engine is a bit tacky, but it functions well, allowing end users to easily navigate content, with fancy tricks like searching - by name or by tags.
Formatting is based on individual EJS files - see ejs.co for more info on EJS templating.
An installation guide can be found at Installation.