I used VEconLab's demand experiment in my sophomore level introduction to environmental and resource economics course this week:
This exercise introduces the notion of demand as a function with "steps" that represent maximum willingness to pay, i.e. the marginal value of consumption. Step function graphs facilitate a subsequent understanding of buyers' surplus, since the area under demand can be calculated visually. ... The exercise is designed to highlight the importance of making decisions at the margin, since purchase decisions based on comparisions [sic] of average values and purchase price will lead to lower earnings. One treatment option involves adding a tax to be paid for each unit purchased, which can lead to a discussion of demand shifts. Learning is enhanced by feedback about relative earnings.
I used it to illustrate demand curves (the course has no prequisite) and consumer surplus. I motivated the tax as a tax on pollution. There were 20 rounds, 10 without the tax and 10 with the tax. I estimated the OLS model with 64 students, 20 rounds (clustered standard errors):
Here are the results for the no tax treatment (round 10):
Here are the results with the tax (=$2) treatment (round 20):