WabiSabi Wiki was an experimental wiki engine in PHP with flat file storage, with the goal of making it as small as possible without sacrificing too many features. It ended up at 275 lines, including a decent markup engine, page backups, anti-spam protection and plugin system.
The engine was developed between 2009 and 2012; last known install was from December 2015. People discussed it, used it, modified it. But my interests have shifted, and I can't afford to worry about yet another live wiki anymore. So here's a snapshot, for posterity. Enjoy!