Effectiveness: Low

Advantages:  Potential of increasing a spammers cost of sending spam.

Limitations: It is possible that the spammer will think the hits are legitimate and if the spammer has user level tracking in the links it could make the spammer think he has a person interested in their spam message which could ultimately lead to more spam messages.

How does it work:  Punish loads the web pages in a spam email message a certain number of times.  Since nothing is being bought from these visits it could minimally increase the bandwidth costs to the spammer without any increase in revenue.