This is not about game development, but about digital photography and image processing:
http://www.cambridgeincolour.com/tutorials.htm
Best damn collection ever, IMHO. Helps a lot to understand issues related to HDR, gamma correction, tone mapping, depth of field, color correction and so on.
Highly recommended to everyone, even if you've read stuff about these topics before. I'd say a lot of game developers themselves should also look into it...