GLSL

See also:

The Art of Texturing Using The OpenGL Shading Language - oZone3D's tutorials for using GLSL for dynamic texturing. Includes environment mapping, glass mapping and alpha mapping.
Create Digital Motion - The weblog offers comments, examples and news regarding GLSL.
3D shaders - Online home of the OpenGL Shading Language (orange) book
GLSL - An Introduction - A quick introduction to GLSL, including code snippets for common shader operations.
GLSL Quick Reference Guide - A PDF chart listing standard GLSL variables and constants.
GLSL Shaders - Introduction - Primer for using GLSL. Includes a gallery and sample code for a fractal noise generator.
GLSL Tutorial - Primer for GLSL beginners: pipeline diagrams, documentation and sample code.
NVIDIA SDK 9.51 Code Samples - nVidia's OpenGL SDK. Includes code, Windows binaries and rendered examples of GLSL.
The OpenGL Shading Language Specification - Detailed documentation on the GLSL language. Includes standard variables, constants and functions. [PDF]
OpenGL Shading Language Tutorials and Demos - ClockworkCoders's guide to GLSL. Includes a gallery, sample code, tips and tricks.
Shadow Mapping Tutorial Using GLSL - Sample renderings and code for shadow mapping.
Soft Shadow Mapping Using Percentage Closer Filter - Compares various approaches to using GLSL for soft shadow mapping and refraction techniques.
Wikipedia: GLSL - A brief history, list of GLSL datatypes and explanation of variable types.