A content provider system interacts with a network of web sites to provide behavior-based content to users. Operators of the web sites add widgets to selected web pages of their sites. The widgets, when executed on the computing devices of users who view the selected web pages, report user-generated events to the content provider system. The content provider system analyzes the reported events to detect behavioral associations between particular web sites, web pages, products, and/or other types of items. The widgets may also retrieve and display behavior-based content that is based on these item-to-item behavioral associations. For example, when a user views a particular web page, a widget on that page may request and display descriptions of, and links to, other sites or pages that are (a) behaviorally related to the page being viewed or an item represented thereon, and/or (b) behaviorally related to the past browsing activities of the particular user.