GLSL

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.
http://www.ozone3d.net/tutorials/glsl_texturing.php

Create Digital Motion
The weblog offers comments, examples and news regarding GLSL.
http://createdigitalmotion.com/tag/glsl/

3D shaders
Online home of the OpenGL Shading Language (orange) book
http://3dshaders.com/

GLSL - An Introduction
A quick introduction to GLSL, including code snippets for common shader operations.
http://nehe.gamedev.net/data/articles/article.asp?article=21

GLSL Quick Reference Guide
Chart listing standard GLSL variables and constants. [PDF]
http://mew.cx/glsl_quickref.pdf

GLSL Shaders - Introduction
Primer for using GLSL. Includes a gallery and sample code for a fractal noise generator.
http://www.davidcornette.com/glsl/

GLSL Tutorial
Primer for GLSL beginners: pipeline diagrams, documentation and sample code.
http://www.lighthouse3d.com/opengl/glsl/

NVIDIA SDK 9.51 Code Samples
nVidia's OpenGL SDK. Includes code, Windows binaries and rendered examples of GLSL.
http://http.download.nvidia.com/developer/SDK/Individual_Samples/samples.html

The OpenGL Shading Language Specification
Detailed documentation on the GLSL language. Includes standard variables, constants and functions. [PDF]
http://www.opengl.org/registry/doc/GLSLangSpec.Full.1.20.8.pdf

OpenGL Shading Language Tutorials and Demos
ClockworkCoders's guide to GLSL. Includes a gallery, sample code, tips and tricks.
http://www.clockworkcoders.com/oglsl/index.html

Soft Shadow Mapping Using Percentage Closer Filter
Compares various approaches to using GLSL for soft shadow mapping and refraction techniques.
http://www.cs.utah.edu/classes/cs5610/projects-2005/lha/

Wikipedia: GLSL
A brief history, list of GLSL datatypes and explanation of variable types.
http://en.wikipedia.org/wiki/GLSL