12. A computer system comprising: one or more computing devices; an interface of the one or more computing devices that is programmed to access information that identifies one or more of: i) a search history that identifies one or more search queries that were previously input to a mobile computing device by a user and search results that are associated with the one or more search queries, and ii) content that was previously requested and displayed on the mobile computing device to the user, wherein the content comprises one or more web pages that were previously displayed on the mobile computing device; a reverse geocoder that is installed on the one or more computing devices and that is programmed to identify a geographic location to which future voice input from the user is likely to relate based at least in part on one or more of: i) the search history that identifies the one or more search queries that were previously input by the user and the associated search results, and ii) the content that was previously requested and displayed on the mobile computing device; and a grammar index that is accessible by the one or more computing devices and that includes an index that correlates geographic locations to a plurality of grammars; a grammar selector that is installed on the one or more computing devices and that is programmed to select a first grammar from among the plurality of grammars based at least in part the geographic location identified by the reverse geocoder, wherein the first grammar includes a vocabulary that is relevant to the geographic location; wherein the interface is further programmed to output information that identifies the first grammar, wherein the outputted information causes a grammar used to analyze voice input from the mobile computing device to be changed to the first grammar.
12. A computer system comprising: one or more computing devices; an interface of the one or more computing devices that is programmed to access information that identifies one or more of: i) a search history that identifies one or more search queries that were previously input to a mobile computing device by a user and search results that are associated with the one or more search queries, and ii) content that was previously requested and displayed on the mobile computing device to the user, wherein the content comprises one or more web pages that were previously displayed on the mobile computing device; a reverse geocoder that is installed on the one or more computing devices and that is programmed to identify a geographic location to which future voice input from the user is likely to relate based at least in part on one or more of: i) the search history that identifies the one or more search queries that were previously input by the user and the associated search results, and ii) the content that was previously requested and displayed on the mobile computing device; and a grammar index that is accessible by the one or more computing devices and that includes an index that correlates geographic locations to a plurality of grammars; a grammar selector that is installed on the one or more computing devices and that is programmed to select a first grammar from among the plurality of grammars based at least in part the geographic location identified by the reverse geocoder, wherein the first grammar includes a vocabulary that is relevant to the geographic location; wherein the interface is further programmed to output information that identifies the first grammar, wherein the outputted information causes a grammar used to analyze voice input from the mobile computing device to be changed to the first grammar. 16. The computer system of claim 12 , wherein the search results for the search queries are aggregated across a collection of search results that were provided to a plurality of users of one or more search services.