1. A method implemented by a data processing apparatus, the method comprising: identifying multiple candidate entities that are associated with a first web resource, each candidate entity being a word or a phrase; obtaining a first entity graph representing relationships between entities associated with resources in a collection of resources, wherein the first entity graph includes multiple nodes, each node representing a different entity associated with a respective resource in the collection of resources, each entity being a word or a phrase, wherein the first entity graph includes edges connecting pairs of nodes, and wherein each of the edges represents that two nodes connected by an edge represent two entities that are frequently associated with a same resource in the collection of resources; filtering the first entity graph to remove nodes that do not represent any of the candidate entities associated with the first web resource; generating, from the filtered first entity graph, a second entity graph for the first resource, including removing nodes from the filtered first entity graph that are not connected by an edge to at least one other node in the filtered first entity graph; identifying candidate entities that are represented by respective nodes in the second entity graph as being central entities for the first resource; generating respective search queries for each of the identified central entities; obtaining search results responsive to the search queries from a search engine; selecting a web resource referenced by a particular search result of the obtained search results as relevant additional content for the first web resource; and associating the relevant additional content with the first web resource for presentation to a user requesting content from the first web resource.