24. The computer program product of claim 21 , wherein two queries that have the same terms and differ only in the order of the terms are counted as the different unique queries.
21. A computer program product encoded on one or more non-transitory computer storage media, the computer program product storing instructions that when executed by the one or more computers cause the one or more computers to perform operations comprising: receiving a search query; obtaining a plurality of search results for the search query from a search engine, wherein each search result identifies a respective search result resource, and wherein the plurality of search results includes one or more search results that identify search result resources that are in a particular site; using a site quality score for the particular site to rank the plurality of search results; and determining the site quality score for the particular site, comprising: determining a numerator that is a function of a first count of textually unique queries submitted to the search engine that have been categorized as referring to the particular site, wherein each such textually unique query is counted once in the first count; determining a denominator that is a function of a second count of textually unique queries submitted to the search engine that have been associated with resources in the particular sites, wherein each such textually unique query is counted once in the second count; and determining the site quality score for the particular site as a ratio of the numerator and the denominator.