According to Wikipedia, Opportunity Cost is "the cost of any activity measured in terms of the value of the next best alternative forgone (that is not chosen)." Metafilter discusses an opportunity cost question that stumps far too many economics students.

Answer this: "You won a free ticket to see an Eric Clapton concert (which has no resale value). Bob Dylan is performing on the same night and is your next-best alternative activity. Tickets to see Dylan cost $40. On any given day, you would be willing to pay up to $50 to see Dylan (because he's so cool!). Assume there are no other costs of seeing either performer. Based on this information, what is the opportunity cost of seeing Eric Clapton? A. $0 B. $10 C. $40 D. $50"

See if you can determine the correct answer before you look it up. There are links to more information in the form of pdfs, and a discussion in the comments. The problem seems to raise more questions that it answers. Link