Simple DirectMedia Layer (SDL), a library for game developers primarily, has finally released version 2.0.0 a few short days ago. Originally developed by Sam Latinga, former lead software engineer at Blizzard and current employee of Valve Software, SDL handles complicated functions such as input devices, video, threading, networking, fonts, and so forth. It complements OpenGL, which is not designed for any of the aforementioned tasks. You can see it used in games such as Telltale's The Walking Dead and Frictional's Amnesia: The Dark Descent
A simple way of understanding it is: as Direct3D gets help from DirectX, OpenGL gets help from SDL.
SDL Logo, Image Credit: Wikipedia
SDL is not a Khronos standard. Prior to the current release, it was licensed under LGPL which required any source code modifications to be shared. Version 2 has been re-licensed under zlib which removes this copyleft requirement. This is advantageous for game developers who wish to modify API while maintaining their secret sauce, increasing adoption, at the expense of potentially fewer contributions.
Latinga's employer, Valve, has interest in a simple and cross platform complement to OpenGL. Valve has been taking Linux and Mac seriously recently. A strong and more permissive SDL helps software portability. SDL is compatible with Windows, Mac, Linux, FreeBSD, iOS, and Android.
The removal of copyleft would also help Valve maintain their own fork of the library without the requirement to share and share alike. Valve was likely not the cause of the switch to zlib, however, as the change was announced during development of SDL 1.3, quite a while before he was hired.
SDL 2.0 was announced the same day as he was hired to Valve Software. This however, at least I expect, was not a coincidence. SDL is available at the project website.