10. 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 in a search engine system a query, the query comprising query text submitted by a user; searching a first collection of resources to obtain one or more first search results, wherein each of the one or more first search results has a respective first search result score; searching a second collection of web resources to obtain one or more second search results, wherein each of the one or more second search results has a respective second search result score, wherein the resources of the first collection of resources are different from the resources of the second collection of web resources; determining from historical user click data that resources from the first collection of resources are more likely to be selected by users than resources from other collections of data when presented by the search engine in a response to the query text; generating enhanced first search result scores for the first search results as a consequence of the determining, the enhanced first search result scores being greater than the respective first search result scores for the first search results; generating a presentation order of first search results and second search results in order of the enhanced first search result scores and the second search result scores; generating a presentation of highest-ranked first search results and second search results in the presentation order; and providing the presentation in a response to the query.
10. 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 in a search engine system a query, the query comprising query text submitted by a user; searching a first collection of resources to obtain one or more first search results, wherein each of the one or more first search results has a respective first search result score; searching a second collection of web resources to obtain one or more second search results, wherein each of the one or more second search results has a respective second search result score, wherein the resources of the first collection of resources are different from the resources of the second collection of web resources; determining from historical user click data that resources from the first collection of resources are more likely to be selected by users than resources from other collections of data when presented by the search engine in a response to the query text; generating enhanced first search result scores for the first search results as a consequence of the determining, the enhanced first search result scores being greater than the respective first search result scores for the first search results; generating a presentation order of first search results and second search results in order of the enhanced first search result scores and the second search result scores; generating a presentation of highest-ranked first search results and second search results in the presentation order; and providing the presentation in a response to the query. 14. The system of claim 10 , wherein generating the presentation of highest-ranked first search results and second search results in the presentation order comprises: generating each of the second search results in a web search results presentation format; and; generating each of the first search results in a different presentation format.