1. A system, comprising: one or more computers configured to: generate search session data and search selection data, wherein: the search session data indicates, for each search session of multiple search sessions that were each performed by a respective user, multiple search queries submitted by that respective user during the search session to search among items offered in commerce in an electronic marketplace; and the search selection data indicates, for each search query, of multiple search queries, that results in a set of corresponding search results comprising a plurality of the items offered in commerce in the electronic marketplace, a particular search result that was selected from that set of corresponding search results by a respective user; perform a co-occurrence analysis on the search session data and the search selection data to generate one or more search indices that specify, for a given search query, one or more search queries determined to be related to the given search query according to the co-occurrence analysis; receive, from a remote client, a search query for one or more of the items offered in commerce in the electronic marketplace; in response to the receipt of the remote client's search query, search the one or more search indices to identify one or more related search queries that are related to the remote client's search query; extract, from search queries obtained from the search session data, a plurality of product features of the items offered in commerce in the electronic marketplace; and provide, to the remote client, a search refinement tool comprising a display of several product features, of the extracted plurality of product features, that are relevant to the remote client's search query, wherein each of the displayed several product features is configured for selection to cause a further display of at least one of: search results generated in response to refining, based on which of the displayed several product features is selected, the remote client's search query; or a refinement of the one or more related search queries that is based on which of the displayed several product features is selected.