Points (Pro & Network versions):
For each product, set the Points and the Redeem (number of point required to receive the product for free).
When a customer buys a product they receive the points. When they accumulate enough points they can redeem the points to receive a product for free (ie. buy 10 get 1 free).
When you go to finish a sale, the software will open a Redemption window that allows you to use the customers point.
You must select the customer before Finishing the sale.