A user interface (UI) in the form of a single navigable window enables a user to navigate to and between multiple different functionalities that are provided by a single application program. Novel use is made of a navigation model that manages the user's navigation activities to and between the different functionalities. Navigation instrumentalities enable the user to navigate among the different functionalities and include links to each of the different functionalities as well as browser-like navigation buttons. Context-sensitive command sets can also be provided along with the single navigable window. The context sensitive command sets include commands that automatically change as the user's computing context changes, e.g. as the user moves from functionality to functionality. The single application can be defined as a software platform that is extensible to receive and incorporate different functionalities. The functionalities can be provided as software modules that can be sent over a network such as the Internet. The extensible software platform provides a basis to offer a subscriber or fee-based service where different subscribers can, for a fee, access different functionalities via a network such as the Internet.