Simple Direct-Media Layer (SDL)
OpenGL only handles graphics (2D and 3D). Sam Lantinga (who now works at Loki Software Inc) wanted to handle !CD-!ROMs, audio, mixers, joysticks, keyboards, MPEG playback, and anything else related to multimedia and games. He also wanted to do all this in a platform independent way so he could compile and use the same source code on Windows, MacOS, Linux, etc. The SDL library achieves this. SDL relies heavily on other libraries like Mesa to do the grunt work. You can think of SDL as a more powerful version of GLUT.