Free, open source game engine online
An easy to use 2D HTML5 game engine written in TypeScript
scene-oriented, flexible 3D engine (C++, Python, C#, Java)
Haskell game engine library for roguelike dungeon crawlers
KlayGE is a cross-platform open source game engine
XNA based game engine completely written in c#
Open source, moddable game engine for the "Polygon Love 2" game
Tactical, turn-based game engine, that uses OpenGL and WinAPI