21. A non-transitory computer-readable medium having stored contents that cause a computing system of a content item selection service to automatically perform a method, the method comprising: receiving, by the computing system, information about search results generated by an online retailer in response to a search request by a user, wherein the search results are to be displayed to the user as part of a single search results Web page and include multiple products from multiple product categories that are available to be acquired from the online retailer, and wherein the content item selection service is separate from the online retailer; automatically determining, by the computing system and before the search results are displayed to the user as part of the single search results Web page, a product category of the multiple product categories to associate with the search request, the determining including analyzing information about the multiple products in the search results and identifying a most frequent product category to which the multiple products belong; automatically identifying, by the computing system, one or more additional products from the determined product category to display to the user along with the search results as part of the single search results Web page, the additional products being distinct from the multiple products in the search results; sending, by the computing system, one or more electronic communications that have information about the identified one or more additional products to include as part of the single search results Web page displayed to the user; determining, by the computing system and based at least in part on the determining of the product category to associate with the search request, to further associate the determined product category with one or more additional Web pages that are displayed to the user as a result of one or more interactions by the user with the search results included in the displayed single search results Web page; and sending, by the computing system, one or more additional electronic communications with information about one or more further products selected from the determined product category to include as part of the one or more additional Web pages displayed to the user.
21. A non-transitory computer-readable medium having stored contents that cause a computing system of a content item selection service to automatically perform a method, the method comprising: receiving, by the computing system, information about search results generated by an online retailer in response to a search request by a user, wherein the search results are to be displayed to the user as part of a single search results Web page and include multiple products from multiple product categories that are available to be acquired from the online retailer, and wherein the content item selection service is separate from the online retailer; automatically determining, by the computing system and before the search results are displayed to the user as part of the single search results Web page, a product category of the multiple product categories to associate with the search request, the determining including analyzing information about the multiple products in the search results and identifying a most frequent product category to which the multiple products belong; automatically identifying, by the computing system, one or more additional products from the determined product category to display to the user along with the search results as part of the single search results Web page, the additional products being distinct from the multiple products in the search results; sending, by the computing system, one or more electronic communications that have information about the identified one or more additional products to include as part of the single search results Web page displayed to the user; determining, by the computing system and based at least in part on the determining of the product category to associate with the search request, to further associate the determined product category with one or more additional Web pages that are displayed to the user as a result of one or more interactions by the user with the search results included in the displayed single search results Web page; and sending, by the computing system, one or more additional electronic communications with information about one or more further products selected from the determined product category to include as part of the one or more additional Web pages displayed to the user. 26. The non-transitory computer-readable medium of claim 21 wherein the search request includes one or more search terms, and wherein the automatic determining of the product category further includes using information about actions previously taken by other users involving products that are part of the determined product category, the previously taken actions occurring after prior search results were provided to the other users for prior searches by the other users that each included at least one of the one or more search terms.