Sophia Learning
Sophia: Price Controls
Lesson uses power point notes and related videos to help students understand ways that the government tries to control prices in an economic system, and what the expected results are.
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
South Western Learning: Econ News: Comparative Statics: Perfect Competition
This site offers economics-based newspaper headlines and asks follow-up questions based on supply and demand in a perfectly competitive market.