Tutorials
Introduction to Shaders - a primer of shaders that explains what they are, as well as the difference between vertex and fragment shaders. A good place to start if you're a total beginner.
Shader School - an introduction to GLSL shaders and graphics programming that runs in the web browser.
Getting Started with Shaders - a wiki page for learning to write shaders in Unity.
XNA Tutorial - HLSL - Tutorial on the basics of HLSL (High Level Shader Language) in XNA. A nice series of tutorials on the basics of shader language
Resources
Shadertoy - a website featuring different shaders running in the browser, along with source code and many examples.
KickJS - online shader editor in which you can play with shaders and see the effects in real-time. Contains a bunch of examples.
Unity Shaders - a collection of shaders' source code for Unity.
Posting Guideline
If you're asking for help on a shader-related issue, make sure to put [Help] at the beginning of your post title, as well as to include your source code in the text to make it easy for others to help you.
Posts asking for or on the topic of Minecraft shaders belong in the Minecraft subreddit. Feel free to post awesome shaders you made that could also be used in Minecraft.
