The Brick Engine is a fast and simple lo-fi gaming engine. The engine provides a comprehensive API, including dazzling visual features, song playback, and collision detection: everything you need to make games with quickness and ease. The engine provides a straightforward C API and has language bindings for Tcl and Pascal, with support for more languages on the way.
openDarkEngine is a from-scratch rewrite of the famous Dark Engine originally written by Looking Glass Studios. The engine was used in the Thief 1, Thief Gold, Thief 2, and System Shock 2 games. This rewrite is targeted at newer machines and should improve compatibility and give the games (and all the fan made material) a never ending life. It should also allow manipulation of the original data files.