EDF Chemical Scorecard http://www.scorecard.org/
Toxic Release Inventory data for the US. Sources and amounts of toxic chemicals released to air and water. Also has information on health effects and other topics.
Smog City http://www.smogcity.com/welcome.htm
Interactive simulator showing how a person's choices affect air quality. Includes information about the ozone, the composition of smog, and air quality index.