Described are computer-based methods and apparatuses, including computer program products, for dynamically enabling customized web content and applications. One or more rules are stored in a database. Default tag code is transmitted to a browser in response to a request from the browser, the default tag code including data that causes the browser to generate a tag module. An update is received from the tag module comprising data indicative of a visitor's interaction with web page content displayed through the browser. A condition associated with a rule from the one or more rules is determined to be satisfied based on the update. An action associated with the rule is performed, comprising transmitting custom targeted tag code to the tag module, wherein the custom targeted tag code includes data that causes the browser, upon execution of the custom targeted tag code by the tag module, to perform an action.