Tutorial for curves adjustment

I just stumbled upon this tutorial, which I found hugely helpful... A walk through of adjusting the curves in Photoshop / LR, which has always seemed like a dark art to me!

How to do Quick and Easy Curves Adjustments in Photoshop

Photo Editor Curve Tool is not exactly a tutorial, but is an explanation of how the Curve works, by showing how Levels and Curves do the same operations.