Shading Languages

GLSL vs Cg
Compares the APIs, workflow and variable types between GLSL and Cg.
http://appsrv.cse.cuhk.edu.hk/~wmpang/tutorials/glsl.html

GPU Shading
GLSL and ARB examples of texture mapping and lighting; includes code examples and screen shots. [PDF]
http://www.csee.umbc.edu/~olano/435/GPUshading.pdf

OpenGL Geometry Shader4 Extension
Specification for OpenGL's geometry shader APIs. Geometry shaders live in between vertex and fragment shaders in OpenGL's pipeline, and are used to introduce new primitives based on incoming vertices. Detailed documentation and pipeline diagram.
http://developer.download.nvidia.com/opengl/specs/GL_EXT_geometry_shader4.txt

OpenGL Shader Designer
A freeware shader development IDE for Windows; aids in developing vertex and fragment (pixel) shaders in GLSL. Includes tools, tutorials, and sample Space Invaders app (requires VC++ 4).
http://www.typhoonlabs.com/

Shader Language Comparison
PDF comparing HLSL, GLSL and Cg. Includes diagrams and code samples. [PDF]
http://www.cs.unb.ca/undergrad/html/documents/Lovesey_Senior_TechReport.pdf

Shader Programming
Dr. Jian Huang's presentation on OpenGL shaders; includes history, overview, rendering examples, and comparisons of ARB vs Cg. [PDF]
http://www.cs.utk.edu/~huangj/CS594F03/shaders/Shader_Programming.pdf

Shader Theory
A tutorial on the basics of shading theory in 3D computer graphics.
http://www.warpedspace.org/tutorials/shading_theory/