12. A computer program product embedded in a computer-readable medium for maintaining an item index able to be used by any search engine able to utilize an item index to generate a search result, the computer program product including instructions that, when executed by a processor, cause the processor to perform steps comprising: determining that, in a distinguished search result produced by the search engine in response to a distinguished query containing a distinguished query term, a user has selected a distinguished item from the distinguished search result; and in response to so determining, updating in an index entry for the distinguished item the contents of an item selection field to reflect a tendency of users to select the distinguished item from search results produced by the search engine in response to queries containing the distinguished query term, the contents of the item selection field including one or more query terms each repeated a number of times representing a level of relevance of each of the one or more query terms to the item, the level of relevance corresponding to the tendency of users to select the distinguished item, the updating including adjusting the number of times at least one of the one or more query terms is repeated in the item selection field, wherein the search engine is able to generate the search results based on the item index including the item selection field by determining an ordered set of items relating to a received query, an order of the ordered set of items being determined at least in part by generating a ranking score for each item in the ordered set of items, the ranking score being generated for each item in the ordered set of items by assigning a field score for each occurrence of one of the one or more query terms in the item selection field and summing the field scores for each item, the search results being generated without modifying operation of the search engine.
12. A computer program product embedded in a computer-readable medium for maintaining an item index able to be used by any search engine able to utilize an item index to generate a search result, the computer program product including instructions that, when executed by a processor, cause the processor to perform steps comprising: determining that, in a distinguished search result produced by the search engine in response to a distinguished query containing a distinguished query term, a user has selected a distinguished item from the distinguished search result; and in response to so determining, updating in an index entry for the distinguished item the contents of an item selection field to reflect a tendency of users to select the distinguished item from search results produced by the search engine in response to queries containing the distinguished query term, the contents of the item selection field including one or more query terms each repeated a number of times representing a level of relevance of each of the one or more query terms to the item, the level of relevance corresponding to the tendency of users to select the distinguished item, the updating including adjusting the number of times at least one of the one or more query terms is repeated in the item selection field, wherein the search engine is able to generate the search results based on the item index including the item selection field by determining an ordered set of items relating to a received query, an order of the ordered set of items being determined at least in part by generating a ranking score for each item in the ordered set of items, the ranking score being generated for each item in the ordered set of items by assigning a field score for each occurrence of one of the one or more query terms in the item selection field and summing the field scores for each item, the search results being generated without modifying operation of the search engine. 13. The computer-readable medium of claim 12 wherein the updating comprises adding the distinguished query term to the contents of an item selection field contained by the index entry for the distinguished item.