SDL
Simple DirectMedia Layer (SDL) is a cross-platform, free and open source software multimedia library written in C that presents a simple interface to various platforms' graphics, sound, and input devices, allowing a developer to write a computer game or other multimedia application that can run on many operating systems including Linux, Syllable, Haiku/BeOS, Windows, Mac OS X, AmigaOS and its clone MorphOS. It manages video, events, digital audio, CD-ROM, sound, threads, shared object loading, networking and timers. (Taken from Wikipedia)
Where to get SDL
SDL is available in source form, as well as in binary packages for all officialy-supported platforms, directly from libsdl.org
A Dreamcast port of SDL is provided in the package of add-on libraries for KallistiOS (kos-ports) from the cadcdev SVN repository on SourceForge
Documentation and tutorials
Official SDL documentation is available on libsdl.org
Tutorials at wiki.gamedev.net
Tutorials at Lazy Foo' Productions
SDL Tutorials at Cone3D Programming