23. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: detecting an occurrence of a pre-search trigger, the pre-search trigger corresponding to a user action to gain access to a search interface at a particular time; identifying a user associated with the pre-search trigger; identifying a topic classified as a likely topic of interest for the identified user based on a browsing history of the identified user or a search query history of the identified user; in response to detecting the occurrence of the pre-search trigger, determining a confidence score associated with the topic based on the particular time of the user action, the confidence score indicating a likelihood that the topic is of interest to the identified user at the particular time; determining that the confidence score associated with the topic satisfies a predetermined threshold value; instructing a search engine to execute a search using a search query associated with the particular topic in response to determining that the confidence score associated with the particular topic satisfies the predetermined threshold value; and providing a representation of a resource identified in results received in response to the search.
23. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: detecting an occurrence of a pre-search trigger, the pre-search trigger corresponding to a user action to gain access to a search interface at a particular time; identifying a user associated with the pre-search trigger; identifying a topic classified as a likely topic of interest for the identified user based on a browsing history of the identified user or a search query history of the identified user; in response to detecting the occurrence of the pre-search trigger, determining a confidence score associated with the topic based on the particular time of the user action, the confidence score indicating a likelihood that the topic is of interest to the identified user at the particular time; determining that the confidence score associated with the topic satisfies a predetermined threshold value; instructing a search engine to execute a search using a search query associated with the particular topic in response to determining that the confidence score associated with the particular topic satisfies the predetermined threshold value; and providing a representation of a resource identified in results received in response to the search. 32. The non-transitory computer storage medium of claim 23 , wherein determining the confidence score, instructing the search engine to perform the search, and providing the representation of the resource identified in the results occur during a web browsing session of the identified user before the identified user enters any query terms related to the topic during the web browsing session.