The Document Object Model (DOM) of a sampled web page is used to create a rule that extracts item-related data from web pages having a similar DOM structure to the sampled web page. In response to a user request for such a web page, the rule is retrieved from a data server based on the page's URL, and is applied to the DOM of the web page to extract item-identifying data. The item is then identified--preferably by the data server--by matching the item-identifying data to an item in a database. Supplemental information about the item is then retrieved from the database and supplied to the user's computer for viewing in conjunction with the requested page. In a preferred embodiment, the rule is retrieved from the data server and applied to the web page by a client application that runs in conjunction with a web browser.