I recently came across a new site (to me) called TutorialBoard.net which is dedicated to "high quality tutorials for graphics design, motion graphics and visual effects artists" and they are true to their word. Great content, great tutorials.
One tutorial that we as photographers will find useful is "Whiten teeth to improve a smile in Photoshop CS4".
It is not uncommon for teeth (and eyes) to appear darker in an image than they actually are in real life and if we want to give our clients the best possible product we need to correct this problem, this tutorial makes the task quick and easy.
But sadly for those with older versions of Photoshop, this tutorial is specific to Photoshop CS4.