1. A computer-implemented method comprising: receiving in a computer a request to run a live appliance, the live appliance providing a computing environment that a user can run on the computer, the live appliance being defined by a data source that includes a version description for a current version of a virtual machine image; determining whether the computer has subscribed to the live appliance, and if not, subscribing to the live appliance on the computer, reading the data source for the live appliance, and using the current version description to initiate downloading of the current version of the virtual machine image to the computer; when the computer has subscribed to the live appliance, detecting whether a change has occurred in the data source that changes the version description designated as the current version description, and if a change has occurred, reading updated data from the data source, the updated data designating a second version description as the current version description, the second version description describing a different second version of the virtual machine image as the current version of the virtual machine image; obtaining the current version of the virtual machine image as described by the second version description; and running the current version of the virtual machine image.