We present an approach for constructing recommender systems that support a user in his navigation through a given Web site. This approach is based on a proxy-like architecture that intercepts the user's requests and the Web site answers, analyzes them in respect to an explicit model of the Web site and some prototypical uses of it and dynamically generates, in a separate window, a tip and/or an access to additional functionalities related to the Web site objective. We illustrate by examples from the construction of a recommender system for a Web site proposed by the French Ministry of Research and Education.