Other
Elemental Economics: Review of Basic Definitions: Competition
This site provides a good description of the four types of market structures: perfect competition, monopolistic competition, oligopoly, and monopoly, along with the major characteristics of each.
Annenberg Foundation
Annenberg Learner: Economics U$a: 21st Century Edition
A micro and macroeconomics course for 21st century learners. 28 half-hour videos sessions with text and guides regarding markets, firms, supply and demand, perfect competition and inelastic demand, economic efficiency, monopolies,...