Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer. It is used by MPEG playback software, emulators, and many popular games, including the award winning Linux port of "Civilization: Call To Power."
Simple DirectMedia Layer supports Linux, Windows, BeOS, MacOS Classic, Mac OS X, FreeBSD, OpenBSD, BSD/OS, Solaris, IRIX, and QNX. There is also code, but no official support, for Windows CE, AmigaOS, Dreamcast, Atari, NetBSD, AIX, OSF/Tru64, RISC OS, and SymbianOS.
SDL is written in C, but works with C++ natively, and has bindings to several other languages, including Ada, Eiffel, Java, Lisp, Lua, ML, Perl, PHP, Pike, Python, and Ruby.
The SDL Guide section is pretty incomplete. If you have anything to add please do so. That is what the wiki is for!