I was hesitant to add this to the list because I don't like it as a practice. It's an ugly business. It is potentially profitable, however, so I'll add it here—although I'll just glaze over it. Basically, you find domain names that are not taken yet and grab them up, or strategically position yourself to take over domain names that are up and running today but whose owners, for some reason, might forget to renew ownership upon expiration. Then, basically, you sit on the domain name and sell it to someone who may have a legitimate use for it for a ridiculous markup.