a game programming library for C/C++ developers distributed freely, supporting the following platforms: DOS, Unix (Linux, FreeBSD, Irix, Solaris, Darwin), Windows, QNX, BeOS and MacOS X. It provides many functions for graphics, sounds, player input, etc..
a library of functions for 2D graphics supporting a superset of the PostScript imaging model, designed to be integrated with graphics, artwork, and illustration programs.
a multi-platform game library for C/C++ developers that provides many functions for graphics, sounds, player input (keyboard, mouse, and joystick), and timers. It also provides fixed and floating point mathematical functions, 3D functions, etc...
a 2D game development library for the Ruby and C++ programming languages, available for Mac OS X, Windows and Linux. (MSVC++ 2005 needed for the Windows C++ version).
Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer.
physics engine written primarily for games. As the name suggests, Box2D is a purely 2D engine. However, Box2D has grown beyond it's humble box simulating roots, and can now handle convex polygons and other shapes coming soon.