11. A system that hosts a site in which content is addressable using Uniform Resource Locators (URLs), the system comprising: a data repository that stores content of the site, including page content corresponding to specific URLs; a server system that provides network-based access to the site content in response to URL requests from user devices, said server system programmed to implement a set of one or more URL rescue strategies for processing requests for invalid URLs, said set of one or more URL rescue strategies comprising a rescue strategy in which the server system: makes a determination, using a selection list, of whether an invalid URL includes at least one term that is suitable for use as a search term for executing a keyword search of site content, wherein the selection list includes at least one of the following: a whitelist, a blacklist, a list of parameters; and when the determination is positive, (1) executes a keyword search of the content of the site using one or more search terms extracted from the invalid URL and an index of the site content; and (2) returns, to a device that requested the invalid URL, a page containing results of the keyword search.
14. The system of claim 11 , wherein the site hosts an electronic catalog, and the keyword search of site content is a catalog search for catalog items associated with the invalid URL.