The de facto standard graphics programming library (application program interface, or API).
The OpenGL Shading Language, or GLSL, or "glslang" for short, is a GPU programming language that is part of the core OpenGL 2.0 specification.

The revolution in graphics programming is shading languages providing programmability over Graphics Processing Units (GPUs). Cg is one such high-level language.

The GUI Toolkit, Framework Page
A web page containing a comparison of many toolkits. There are a lot of them out there, below are three of the best for interactive graphics programming.

A simple, no-frills (no logo even! :) C GUI used to simplify learning graphics programming. It makes window creation fairly easy, but it lacks a lot of nice features such as buttons, file browsers, etc. If you're going to learn interactive graphics, you might as well also learn to use a more powerful GUI toolkit.

A powerful C GUI used to create The Gimp. It's somewhat hard to learn (lacking good manuals), but once you get comfortable with its syntax and methodology, you'll find it to be a very good toolkit.


The gdk-pixbuf Library

An object-oriented C++ GUI used to create KDE. There's some documentation here.

An easy C++ GUI based on the old Forms package. Includes a visual GUI creator program (fluid) which lets you set up buttons, windows, etc., and generate the skeleton callback C++ file. Using OpenGL is simplified by creating a special window subclass Fl_Gl_Window.