Wikimedia
Wikipedia: There Ain't No Such Thing as a Free Lunch (Tanstaafl)
This on-line encyclopedia provides the definition of the economic acronym "TANSTAAFL," which is a way to explain opportunity cost.
Other
Opportunity Cost
This site provides a definition, examples, and applications for the concept of "opportunity cost."
Other
East Tennessee State University: Explicit and Implicit Costs
This resource provides a good example of the differences between explicit costs and implicit costs using the ownership of a pizza restaurant as an example.
Other
Amos Web Gloss Arama: Opportunity Cost
Provides readers with a definition and explanation of the concept of opportunity cost. Gives great details and compares the term with similar concepts.
Tutor2u
Tutor2u: Opportunity Cost
This description explains the concept of opportunity cost and gives examples.