Orx: Portable Game Engine
GemRB is a portable open-source implementation of the Infinity Engine
Game engine supporting many 3D/2D formats and graphic effects
Low-Level Game Engine for Retro Platforms
Portable game engine with main focus on point-and-click adventures
scriptable game engine with built-in support for 2D graphics and sound
Multi-platform game engine similar to old Megaman with a stage editor
A generic 2D engine for any kind of game