It must be one where you can get away with usage-based pricing, and where there is a strong rationale for making it cheap, yet not free. Products that get used up as they are used naturally have this property. If the basic economics are usage-based, then the payment can be too. This class does not include journalism or, indeed, most intellectual property. Yet, this is where micropayments are most often suggested.
Films are a good example.
This price is high enough to discourage many people from seeing the film. There it picks up a few repeat customers, but mostly new ones. Technically speaking, this is a repeated auction. However, there is also the Dutch auction, where prices start high and go lower until somebody bites. Movies are sold to the audience via a very slow Dutch auction, where each phase between price drops can last weeks or months.