1. A system comprising: a client system implemented on one or more computers for use by a plurality of users; wherein the client system comprises: result selection logs stored on memory devices of the client system; and a tracking component that is operable to store in the result selection logs information about search result selections made by users of the client system; wherein the client system is programmed with instructions that are operable, when executed on the client system, to cause the client system to perform operations to: submit a search query to a search engine that receives search queries from the client system and from other sources of queries; receive search results responsive to the search query, the search results including either an initial ranking of the search results from the search engine or actual information retrieval scores for the search results from the search engine; compute, for each of the search results, a respective measure of relevance based on the information stored by the tracking component of the client system about search result selections made by multiple users of the client system and not by other users of the search engine; and re-rank the search results returned by the search engine based on the respective measures of relevance and provide the re-ranked search results in a response to the search query.