10. A system comprising a processor and a computer storage medium coupled to the processor, the computer storage medium having stored thereon a set of instructions, which when executed on the processor cause the processor to: receive a first search query from a requestor; receive metadata from the requestor that identifies a particular custom search engine; access stored configuration information for the particular custom search engine that is identified by the metadata; determine a first corpus to search, the determining the first corpus to search is based on the configuration information, wherein the first corpus is structured content having one or more fields; search the first corpus using the first search query to obtain first search results that match the first search query, the first search results include a plurality of search result items, the search result items each having data for a first field of the fields; determine a second corpus to search, the determining the second corpus is based on the configuration information; generate sub-queries for respective ones of the search result items, the generating sub-queries is based on the configuration information that identifies the first field, wherein the generating the sub-queries includes forming a sub-query for a respective one of the search result items from data in the first field for the respective search result item; search the second corpus using the sub-queries to obtain second search results that match the sub-queries; generate a search result set from the first search results and the second search results, the generating a search result set is based on the configuration information, the generating the search result set includes augmenting respective ones of the search result items from the first search results with a portion of the second search results obtained by the sub-query for the respective search result item from the first search results; and provide the search result set to the requestor.
10. A system comprising a processor and a computer storage medium coupled to the processor, the computer storage medium having stored thereon a set of instructions, which when executed on the processor cause the processor to: receive a first search query from a requestor; receive metadata from the requestor that identifies a particular custom search engine; access stored configuration information for the particular custom search engine that is identified by the metadata; determine a first corpus to search, the determining the first corpus to search is based on the configuration information, wherein the first corpus is structured content having one or more fields; search the first corpus using the first search query to obtain first search results that match the first search query, the first search results include a plurality of search result items, the search result items each having data for a first field of the fields; determine a second corpus to search, the determining the second corpus is based on the configuration information; generate sub-queries for respective ones of the search result items, the generating sub-queries is based on the configuration information that identifies the first field, wherein the generating the sub-queries includes forming a sub-query for a respective one of the search result items from data in the first field for the respective search result item; search the second corpus using the sub-queries to obtain second search results that match the sub-queries; generate a search result set from the first search results and the second search results, the generating a search result set is based on the configuration information, the generating the search result set includes augmenting respective ones of the search result items from the first search results with a portion of the second search results obtained by the sub-query for the respective search result item from the first search results; and provide the search result set to the requestor. 11. A system as recited in claim 10 , further comprising instructions that cause the processor to: provide an interface for a designer to design the particular custom search engine; receive information in the interface that defines the particular custom search engine; store the information received in the interface; and associate the information received in the interface with the particular custom search engine.