Shadertoy: Test your shader online.

If you want to learn and test 3D shader technology, then you can try Shader Toy v0.3 .
This web site allows you to write shaders and view the result .
It also comes with some useful examples .
More can be found on website .

Allegro 5 library for game development.

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.