Paper on passive information gathering
TechicalInfo.Net is an excellent resource for Web Security information. Gunter Ollmann has provided a set of great papers, observations, and links to information gathering tools available on the Internet. The latest addition to this collection is a Passive Information Gathering paper. In the paper he summarizes the techniques many seasoned security professionals use every day, in a tutorial-style, step-by-step document. It is a great read, even if you do use these techniques every day (it is guaranteed that you will learn something new).