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, which can be viewed at cadcdev.svn.sourceforge.net.
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