Multi Theft Auto is a game engine
Free, open source game engine online
A game framework written with osu! in mind.
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
GemRB is a portable open-source implementation of the Infinity Engine
Orx: Portable Game Engine
Game engine supporting many 3D/2D formats and graphic effects
A game engine for console based text games
Vivid3D is an open-source 3D engine with powerful 2D UI.
Spring is a free RTS engine developed for Windows, Linux and Mac OS X.
Data-oriented and data-driven game engine written in Rust
KlayGE is a cross-platform open source game engine
scriptable game engine with built-in support for 2D graphics and sound
online 4X browser game engine
XNA based game engine completely written in c#
A roguelike game engine/editor
TR Game Engine allows you to develop your game quickly and easily.
Tactical, turn-based game engine, that uses OpenGL and WinAPI
Open source, moddable game engine for the "Polygon Love 2" game