Graphical

InRelief, a graphics engine for volumetric pixel art

Text-based

The SHTAP game engine, a.k.a. the SHell Text Adventure Player
 
(Screenshot of a text-based game designed to resemble an open book.)
Adventure Prompt: next-gen authoring system that brings point-and-click gameplay to text adventures
Ramus 2, an interactive fiction authoring system that makes CYOA out of Org Mode files.

Legacy experiments