WebGameGear is a small toolkit and library to program web games with PHP5.
Wintermute Engine Development Kit is a set of tools for creating and running graphical “point&click” adventure games, both traditional 2D ones and modern 2.5D games (3D characters on 2D backgrounds).
2D Game Engine, based on OpenGL and focused on Isometric Games
Open2GE (2 - dimensional Game Engine) is a universal game engine, with a focus on isometric games. Based on OpenGL & SDL, it supports a wide range of features, useful for simple 2D games or complex isometric games, from simple image loading to the rendering of complex scenes, path finding, automatic navigation mesh generation and more. Besides OpenGL and SDL, Open2GE makes use of TinyXML and boost.