This article will help you to understand the concept of sharpness. It features a list of the most common sharpening techniques in Photoshop and GIMP, as well as links to complex tutorials online.
Fotoflexer is a free online photo editor that does not offer only an attractive and intuitive interface and a set of basic functions, but also some advanced features comparable to those offered by expensive professional software.
An introduction to catchlights, attempting to answer a few basic questions like: "What are they?", "Why are they important in digital photography?" and "How can we make the best of them?"