14. A server system comprising one or more computing devices, said server system programmed to respond to a URL (Uniform Resource Locator) request from a user device by a process that comprises: identifying a URL specified by the URL request, said URL including an address of a hosting site and including a character string referencing a content location; determining that the URL specified in the URL request is invalid by determining that the character string does not reference a valid content location of the hosting site; in response to determining that the URL is invalid, executing a URL rescue process that comprises a plurality of URL rescue strategies, including: a first URL rescue strategy that attempts to correct the invalid URL through URL modification, wherein the first URL rescue strategy attempts to modify the invalid URL to produce a valid URL that references a valid content location of the hosting site; and a second URL rescue strategy that comprises (i) extracting from the invalid URL a set of one or more search terms for executing one or more searches of content of the hosting site, wherein extracting the set of one or more search terms comprises using delimiters in the invalid URL to identify a substring of the invalid URL, and comparing the substring to entries in a selection list, said extracting resulting in a selection of less than all text of said character string for use in executing the one or more searches, (ii) executing the one or more searches of content of the hosting site using the one or more extracted search terms, wherein executing the one or more searches comprises applying the one or more search terms to an index of the content of the hosting site, and (iii) incorporating results of the one or more searches into a page to provide to the user device in response to the URL request.
14. A server system comprising one or more computing devices, said server system programmed to respond to a URL (Uniform Resource Locator) request from a user device by a process that comprises: identifying a URL specified by the URL request, said URL including an address of a hosting site and including a character string referencing a content location; determining that the URL specified in the URL request is invalid by determining that the character string does not reference a valid content location of the hosting site; in response to determining that the URL is invalid, executing a URL rescue process that comprises a plurality of URL rescue strategies, including: a first URL rescue strategy that attempts to correct the invalid URL through URL modification, wherein the first URL rescue strategy attempts to modify the invalid URL to produce a valid URL that references a valid content location of the hosting site; and a second URL rescue strategy that comprises (i) extracting from the invalid URL a set of one or more search terms for executing one or more searches of content of the hosting site, wherein extracting the set of one or more search terms comprises using delimiters in the invalid URL to identify a substring of the invalid URL, and comparing the substring to entries in a selection list, said extracting resulting in a selection of less than all text of said character string for use in executing the one or more searches, (ii) executing the one or more searches of content of the hosting site using the one or more extracted search terms, wherein executing the one or more searches comprises applying the one or more search terms to an index of the content of the hosting site, and (iii) incorporating results of the one or more searches into a page to provide to the user device in response to the URL request. 16. The server system of claim 14 , wherein the plurality of URL rescue strategies further includes a third URL rescue strategy that comprises: extracting from the invalid URL an item identifier that identifies a first item; looking up in a mapping table a second item that is related to the first item; and returning to the user device, in response to the URL request, a description of the second item.