Wikipedia
ClanLib
ClanLib is a cross-platform C++ game SDK, currently supporting Microsoft Windows, Mac OS X, and Linux, with partial support for mobile platforms. It has full hardware accelerated graphics support through OpenGL, and also a software renderer. ClanLib also helps in playing sound, using the Vorbis or MikMod libraries, and has classes for collision detection, GUIs, XML, networking, and other things that may be helpful to a game programmer.