A computing system is configured to execute a computer program on a server and to provide a video stream of the program output to a geographically remote client over a communication network. An add-on manager is provided to facilitate the use of add-ons to extend the functionality of the computer program. The add-on manager is responsive to commands received from the client and is configured to associate individual add-ons and add-on data with specific user accounts. The add-ons can be located on the server or some other location remote from the client.