14. A computer program product encoded on a non-transitory computer-readable medium, operable to cause a data processing apparatus to perform operations comprising: for a first document that is included in first search results responsive to a first user-submitted query, selecting a plurality of previously submitted queries for which the first document was a responsive search result, wherein the selected previously submitted queries are selected using a document-to-query-to-document model that associates the first document to the plurality of previously submitted queries and that associates each of the plurality of previously submitted queries to one or more of second documents for which each of the one or more second documents was a responsive search result; for each of the selected previously submitted queries, determining whether there is at least a threshold level of diversity between the first search results and second documents identified as being relevant to the selected previously submitted query, wherein second documents are determined to be relevant to the previously submitted query based on data that is indicative of user behavior relative to the second documents as search results for the previously submitted query; based on the determination of whether there is at least a threshold level of diversity between the first search results and the second documents, identifying one or more queries from the selected previously submitted queries to provide as first suggested queries; and providing the one or more of identified queries as the first suggested queries with the first search results for the first user-submitted query.
14. A computer program product encoded on a non-transitory computer-readable medium, operable to cause a data processing apparatus to perform operations comprising: for a first document that is included in first search results responsive to a first user-submitted query, selecting a plurality of previously submitted queries for which the first document was a responsive search result, wherein the selected previously submitted queries are selected using a document-to-query-to-document model that associates the first document to the plurality of previously submitted queries and that associates each of the plurality of previously submitted queries to one or more of second documents for which each of the one or more second documents was a responsive search result; for each of the selected previously submitted queries, determining whether there is at least a threshold level of diversity between the first search results and second documents identified as being relevant to the selected previously submitted query, wherein second documents are determined to be relevant to the previously submitted query based on data that is indicative of user behavior relative to the second documents as search results for the previously submitted query; based on the determination of whether there is at least a threshold level of diversity between the first search results and the second documents, identifying one or more queries from the selected previously submitted queries to provide as first suggested queries; and providing the one or more of identified queries as the first suggested queries with the first search results for the first user-submitted query. 17. The computer program product of claim 14 , the operations further comprising, for each of the selected previously submitted queries, determining a level of term diversity for the previously submitted query in comparison to the first user-submitted query; where identifying the one or more queries from the selected previously submitted queries to provide as first suggested queries is further based on the determined level of term diversity for each of the selected previously submitted queries.