This program models the creation of money in an economy through a private banking system. People exchange money and either deposit their extra money in the bank, or borrow it if they need to. The model demonstrates that the bank's reserve ratio -- how much money it must keep on hand compared to how much it can loan out -- is the key determiner of how much money is created in the system.
For detailed instructions on using the program, take a look at the info window from the Bank Reserves model.
Click on one of the pictures to see a quicktime movie of the model: