Mahasona engine is an open source game engine.
A generic 2D engine for any kind of game
2D XNA Game Engine
2D - 3D game engine
Open source, moddable game engine for the "Polygon Love 2" game
Java game engine for shoot 'em ups, using Java2D.
Framework for a graphical point and click adventure game in Java
A complete 2D Game Engine with built in drag and drop editor
Cross-platform 2D game engine
Gomoku Game: Playing with Computer