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.
AzureGE is a multi-platform OpenGL and OpenAL-based game engine. It has built-in support for I/O, threading, and networking, a particle system, a material generator, a texture generator, and a mesh/terrain generator. It is intended mainly for creating role playing games, but can also be used for other types of game.
GammaMOO is a fork of the LambdaMOO server with numerous cleaned-up patches and modern MOO functionality. This currently includes multicore support, object caching and other acceleration features, enhanced file I/O, UTF support, new built-in data types, and stability improvements with 64-bit support in progress.