SDL - Simple DirectMedia Library, also known as "libSDL" is an LGPL'd cross-platform library typically used for writing games. It provides a C-based API to control audio and video, as well as providing access to OpenGL, access to CDROM's, a timing system, get input from the keyboard, pointer and joystick, as well as do multi-threaded programing.
Originally written by Sam Latinga, SDL was used for Loki Software GNU/Linux game ports.
The SDL video API is extremely simple and flexable, requiring the modification the image data directly to perform anything more then a blit operation. Luckily, due to SDL's popularity, lots of add-on libraries to SDL have been created to deal with the simplistic nature of the SDL API, giving the ability to load image files, render text, basic drawing operations, etc. You can find a list of add-on libraries and more at the SDL homepage. The image data format used is also changeable, allowing SDL to easily work alongside other libraries.