The CG_GL engine, available here, has got some nice new features like portals (similar to the “Portal” game), bokeh rendering, and PhysX support, to go with all the old features like scattering, particles, volumetric fog, and more.  While you may not want to actually use it for a game, it’s a great educational tool on how to implement these effects in your own projects.

CG_GL engine is OpenGL API-based open source engine for creating and managing effects. The newest version of the engine adds features like movable and scalable portals (like in the “Portal” game) working with NVidia PhysX, improved bokeh depth of field, more realistic fog, particles, fake raytrace using cube maps and many improvements. It now works both on Windows and Linux. There is also simple game created using the engine.

via CG_GL engine adds new features like portals, bokeh depth of field and particles – opengl.org news.