Processing a browser request that specifies a destination network resource generally includes intercepting a browser request that specifies a selected destination network resource and redirecting the browser request to a network server that differs from the destination resource specified by the browser request. Processing a browser request also may include intercepting a browser request received from a client computer at a proxy server when the browser request specifies a selected destination network resource and performing instructions associated with and in addition to instructions performed to download the selected destination network resource.