GZDoom's music system as a standalone library

Free 2D graphics engine with raster effects for retro/classic style game development

Comprehensive map editor for Doom, Heretic, Hexen and Strife based games mainly geared towards GZDoom source port. Based on Doom Builder 2 by CodeImp (http://www.doombuilder.com/).

The Eternity Engine

GZDoom is a feature centric port for all Doom engine games, based on ZDoom, adding an OpenGL renderer and powerful scripting capabilities

It's a Doom editor

Highly moddable Doom engine with support for all Doom engine games

Open Source real-time strategy game engine for early Westwood games such as Command & Conquer: Red Alert written in C# using SDL and OpenGL. Runs on Windows, Linux, *BSD and Mac OS X.

OpenMW is an unofficial open source engine reimplementation of the game Morrowind.