Since building a game is not about 2D anymore, I just want to build a list of the (not necessarily best, but good enough) open source software available to make games.
I prefer to put emphasis on libraries that insist on specializing on one part of what makes a game (like Ogre does for graphics, and OpenSteer does for steering), rather than engines/libraries that try to feature a lot of different features.
- IrrKlang http://www.ambiera.com/irrklang/
- OpenSteer http://opensteer.sourceforge.net/
- NxOgre (plugin for Ogre3d) http://sourceforge.net/projects/nxogre/
Feel free to update this list and/or add categories.