1. A computer-implemented method for generating a universal query result set, comprising: under control of one or more computer systems configured with executable instructions, receiving a query from a searching entity; submitting the query to a plurality of search indexes, each search index corresponding to a respective category of items, at least one of the plurality of search indexes utilizing at least one of a different ranking property, scale, function, or definition for ranking items relative to other search indexes; receiving one of a plurality of search index result sets from each of the plurality of search indexes in response to the query; determining a plurality of appropriateness scores each corresponding to one of the plurality of search indexes, each of the plurality of appropriateness scores indicating an appropriateness of the category of items corresponding to the respective search index with respect to the query and being based at least in part on historical queries similar to the query that were submitted to the respective search index; determining a universal item score for each of a plurality of items in the plurality of search index result sets at least in part by normalizing the at least one of the different ranking property, scale, function, and definition to a ranking scale common to all the search index result sets; for each of the plurality of items, determining a probability that the item satisfies the query based at least in part on the appropriateness score for the search index associated with the item and the universal item score for the item; including in the universal query result set ones of the plurality of items selected in an order based at least in part on the probabilities of the plurality of items satisfying the query; and providing the universal query result set to the searching entity, wherein determining the plurality of appropriateness scores comprises modifying the plurality of appropriateness scores differently based at least in part on different types of recorded actions associated with the historical queries that were submitted to a corresponding search index.