If you need to calculate DOF, Parallax or Field of View for your photography needs, you may consider the page "Photography Calculators". Unfortunatelly it hard to take it with you in the field, when you need it most.

Another interesting topic of discussion - importance of precise focusing. The article called "The Flawed State of Photography as Art and Documentation" talks about how even a slight shift of focus can change the expression of the person (subject of the photograph) and how hard is it to achieve. At this website there are a few more articles that explore the focusing issues in photography. The posted articels somewhat old and may not reflect the current situation, that's why you should take it with a bit of salt :). The articles are the following:

• A Calamity in the World of Art (Judging Artworks by Slide Reproductions)
• Why No Camera Can Focus
• Focusing Tests
• The Flaw in All Auto-Focusing Cameras: the Messraster Focal Point Exact Screen
• Why the Most Important Invention in Photography Is Not Available