There's a new tutorial...
Improving Image Quality with Lens Corrections
These can make an enormous difference with image quality -- especially with compact cameras and inexpensive lenses. For best results though, it's something that should be used with nearly every camera.
This tutorial includes a mix of technical and applied topics. It also tries to provide a basis for why each type of imperfection can happen without resorting to lots of lens diagrams. Even if you're already familiar with topics like distortion and vignetting, also take a look at the CA section for an overview of the different types. For example, did you know that you can change the visibility and color of axial CA by slightly front or back-focusing your lens?
As usual, please let me know if you feel anything is unclear, if you notice any typos, or just want to add something from your own experience.