I recently gave a presentation on the landscape of open source software in computer games at the Univ. Rey Juan Carlos, where I am currently visiting the Libresoft research group. My slides are available here.
While much of the talk covered well-known libraries (SDL, OpenAL), game engines (Ogre, Irrlicht), physics engines (Bullet, Tokamak), and content creation tools (Blender, GIMP), there were a few surprises. One was how many open source game-creation systems I found (4, more than the zero I expected). These are Game Editor (2d with export to some mobile devices), Construct (2d, some 3d), Novashell (2d), and Sandbox (3d). Another surprise was the game Yo Frankie! (pictured above), which has very high quality animation and artwork, and was produced using Blender.
About the author: Jim is Professor and Chair of Computer Science at UC Santa Cruz. He has research interests in procedural level generation for computer games, as well as automatic bug prediction. His favorite games are Radiant Silvergun and Civilization IV.