Econoclass
What's Wrong With This Picture?
A variety of pictures are provided to help study many economic concepts. Could be a good classroom activity.
Auburn University
Auburn University: A Glossary of Political Economy Terms: Externality
Dr. Johnson offers a very good description of an externality, both positive and negative, and explains how taxes and subsidies can provide the greatest amount of utility to society when confronted with an externality.
Other
The Ludvig Von Mises Institute: The "Externalities" Argument
This article provides an inquiry into the meaning of an externality, and why it is important to the study of economics. (Published October 27, 2003)
Khan Academy
Khan Academy: Externalities
Practice what you've learned about positive externalities and negative externalities in this exercise.
Lumen Learning
Boundless Economics: Introducing Market Failure
An explanation of the causes of market failure and possible governmental responses to such failure.