1. A computer-implemented method of determining a reputation score for an agent, the method comprising: receiving multiple digital content items in a computing system comprising one or more computers, each of the received digital content items being associated with a digital signature that associates the digital content item with a respective agent, the respective agent being one of multiple agents, each of the multiple agents being associated with at least one of the received digital content items; identifying in the computing system a first agent of the identified agents and a first group of the received digital content items, each of the digital content items in the first group being associated by digital signature with the first agent, wherein at least one of the digital content items in the first group is a first portion of a web page, the web page including at least a second portion that is not associated with the first agent; identifying in the computing system one or more second agents of the identified agents, each of the second agents being different from the first agent, each of the second agents having a reputation score, each of the second agents being associated by digital signature with a received digital content item that includes a link to at least one of the digital content items in the first group, wherein at least one of the second agents is associated by digital signature with a received digital content item that includes a link to the digital content item of the first group that is the first portion of the web page; and calculating in the computing system a reputation score for the first agent that is a function of the respective reputation scores of the second agents, wherein each of the digital signatures associated with the digital content items of the first group further includes a context that specifies a subject of the associated digital content item, and wherein the reputation score for the first agent is further a function of the contexts.