16. A system, comprising: one or more computers; an augmentation query store storing augmentation queries; and software stored in a computer readable medium and comprising instructions executable by the one or more computers to cause the one or more computers to perform operations comprising: receiving a query from a user device as input for a search operation that uses the received query to identify search results responsive to the received query; determining a transformation cost score for each of a plurality of candidate augmentation queries for the received query, the transformation cost score being based on a transformation cost value for each of one or more terms of the received query, the transformation cost value for a term of the received query being proportional to a term weight that measures an importance of the term in the received query; selecting a candidate augmentation query from among the candidate augmentation queries based on the transformation cost score of the selected candidate augmentation query, the selecting comprising: comparing each term of the received query to a set of non-transformable terms, each non-transformable term of the set of non-transformable terms being a term for which a matching term in the received query should not be transformed to another term; identifying a proper subset of candidate augmentation queries from among the candidate augmentation queries, each candidate augmentation query identified for the proper subset being identified in response to a determination that the candidate augmentation query includes each term of the received query that matches a non-transformable term; ranking the proper subset of candidate augmentation queries according to the transformation cost score of each candidate augmentation query of the proper subset; and selecting the candidate augmentation query from among the proper subset of candidate augmentation queries based on the ranking of the proper subset; identifying search results responsive to the selected candidate augmentation query; augmenting the search results responsive to the received query with the search results responsive to the selected candidate augmentation query to form an augmented set of search results; and providing the augmented set of search results to the user device as a set of search results responsive to the received query.
19. The system of claim 16 , wherein the operation of providing the augmented set of search results to the user device as a set of search results responsive to the received query comprises the operation of providing visual indicator data to the user device that causes the user device to display visual indicators that differentiate the search results that are responsive to the selected candidate augmentation query from the search results that are responsive to the received query.