University of Illinois
University of Illinois Extension: Ww2010: Atmospheric Pressure
This site from the University of Illinois describes what air pressure is and how it is measured. Explains high and low pressure areas, isobars, and how pressure changes with height.
NASA
Nasa: How Air Pressure Affects You
This site from NASA offers a general overview of air pressure. Explores the concept through experiments and interactive games, as well as discussion questions.