The ioquake3 community effort to continue supporting/developing id's
Xash3D FWGS engine
A .NET ascii/ansi console engine written in C# for MonoGame and XNA
Open source clone of the Age of Empires II engine
A dead simple 2D game engine for Go
Free game engine for development of desktop, mobile, and web games
An OpenGL RTS game engine written in C
Injectable LUA scripting system, SDK generator, live property editor
Game engine to build text adventures.
PyCAPGE - Python Classic Adventure Point and Click Game Engine
A game engine for console based text games
a UCI chess engine
A PICO-8 demake of the SCUMM engine
2D Game Engine
RPG dice roll simulator for vintage Tandy TRS-80 100/102/200 laptops
A 2D game engine and level editor
Multi-platform game engine similar to old Megaman with a stage editor
A small text based game engine (VB.Net)
Open Source 3D Game Engine
TurboSphere is an easy to learn and powerful JS based game engine.
A 2D java game engine
A truly extensible cross-platform game engine
Little game background framework in java.
"Abysmal Depths" is a simple C++-based text adventure game engine.