5. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a search query submitted by a user of a client device; receiving, from the client device, context information that includes a network address of a document that a user of the client device has associated with the search query; for each of multiple topics, determining a first score that reflects an extent to which the topic is indicated as relevant to the context information that includes the network address of the document that the user of the client device has associated with the search query; selecting a particular topic, from among the multiple topics, based on the first scores; selecting a set of search results from multiple different domains that are identified as relevant to the search query; for each search result of the set, determining a second score that reflects an extent to which the particular topic is indicated as relevant to the search result; selecting one or more particular search results from the set of search results based on the second scores; and providing, for output, a search results page that identifies the one or more particular search results.
5. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving a search query submitted by a user of a client device; receiving, from the client device, context information that includes a network address of a document that a user of the client device has associated with the search query; for each of multiple topics, determining a first score that reflects an extent to which the topic is indicated as relevant to the context information that includes the network address of the document that the user of the client device has associated with the search query; selecting a particular topic, from among the multiple topics, based on the first scores; selecting a set of search results from multiple different domains that are identified as relevant to the search query; for each search result of the set, determining a second score that reflects an extent to which the particular topic is indicated as relevant to the search result; selecting one or more particular search results from the set of search results based on the second scores; and providing, for output, a search results page that identifies the one or more particular search results. 8. The system of claim 5 , wherein selecting one or more particular search results from the set of search results based on the second scores comprises: determining whether each of the search results are associated with a second score that exceeds a predetermined threshold; and selecting each of the search results that are associated with a second score that exceeds the predetermined threshold.