BigBruin has posted an introduction to case modding. Here's a short snip:
Go back just a few short years and the appearances of pretty much all computers were slight variations of the same theme. Boring beige boxes dominated the industry, and people looking to express their individuality with the style of their computer had to take matters into their own hands. The term "modding" may not even be a real word (I know MS Word's spell checker is upset with my use of it), but it is appropriate for loosely describing the broad field of modifying a computer system to give it a personalized style. This tip will proceed by looking at some of the basic items, tools, and accessories used in modding.