Allegro 4 and Allegro 5 are cross-platform, open source, game programming libraries, primarily for C and C++ developers.The Allegro library provides low level routines like input, graphics, midi, sound effects, and timing.
It is cross platform : Windows, OS X, Linux, DOS, and others.
The official website is here.
You can find some tutorials, and various articles for beginners.
Other information can be found here.
