3D game engine/framework in C, with Luajit and Python bindings now
Game engine supporting many 3D/2D formats and graphic effects
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
Full featured MUD server with all the trimmings.
A game engine for console based text games
A Tile-based Game Engine written in C# mimicing retro consoles.
Old-school 2D game engine
A cross platform Lua based game engine
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
Flexible and easy-to-use 2d game engine
Multi-platform game engine similar to old Megaman with a stage editor
A game engine written in the D programming language
A 3D game engine in java.