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.
Library of Economics and Liberty
Concise Encyclopedia of Economics: Public Goods
This article describes the relationship between externalities, private and public goods, and government intervention.
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
Public Goods
This article discusses public goods, the free-rider problem, externalities, property rights, and game theory strategy.
Other
National Bureau of Economic Research: Measuring Positive Externalities [Pdf]
This research paper describes how the installation of an anti-theft device on cars has helped reduce crime in general in some areas. The concept of positive externalities can be seen in this example.