SDL based generic 2D game engine
Vivid3D is an open-source 3D engine with powerful 2D UI.
2D ARPG Engine
Mahasona engine is an open source game engine.
Free strategy browsergame engine.
TurboSphere is an easy to learn and powerful JS based game engine.
Cross-platform reimplementation of AMOS BASIC (runtime)
AMOS BASIC reimplementation in Java
A 2D Java game engine