21. The system of claim 19 , wherein the update handler code generates an overlay display object which is part of the web page, and displays the supplemental content in the overlay display object, such that the overlay display object and supplemental content are displayed without opening a new browser window.
19. A server system for dynamically supplementing web pages of web sites, the system comprising: a content server that serves content in response to requests from browsers running on user computing devices, including supplemental content that is adapted for display in web pages that are served by sources other than the content server, said content server comprising computer hardware; and a repository of update handler code that is adapted to be transmitted to, and executed by, the browsers in connection with the web pages; wherein the update handler code, when executed as part of a web page loaded by a browser running on a user computing device, is operative to cause the browser to (a) analyze content of the web page loaded by the browser to detect that the web page includes a link that matches a predefined link signature of a catalog page, and to obtain, from said link, an identifier of a product, (b) retrieve supplemental content associated with the product from the content server, and (c) supplement the web page with the supplemental content associated with the product, said supplemental content including catalog content associated with the product.