Jay just pointed me to Day 22 of "24 Ways to Impress Your Friends" Photographic Palettes. It's a great brief "how-to" article by Dave Shea on how to use photographs as the basis for design on the web. I especially like the section on "Tonal Variations" where he describes how he creates a color pallette using black and white layers with different opacity in Photoshop.

The other articles in the 24 Ways Series are useful and interesting as well. Each has a different author, and teaches you something about design or web development in a very concise and easy-to-learn way.