1. A method performed by a user computer upon loading a web page of a web site, said web page including an update handler tag, the method comprising: at least partly in response to the update handler tag, loading an update handler over a network, said update handler comprising browser-executable code that is loaded as part of the web page by a browser running on the user computer; and via execution of the update handler by said browser running on the user computer: (a) detecting, via an analysis of content of the web page, a reference in the web page to a product represented in an electronic catalog of products, said reference comprising a product identifier included in a link that points to a catalog page associated with the product, wherein detecting the reference comprises determining whether the link matches a pre-specified link signature of a catalog page; (b) retrieving, over a network from a content server that is separate from the web site, supplemental content, including catalog content, associated with the product; and (c) in response to a mouse-over event involving the product, causing the user computer to display the supplemental content in an overlay display object added to the web page.