1. An interoperability network configured to facilitate messaging and mediate policy differences among a plurality of independent service providers and users having associated client machines, the interoperability network comprising one or more computing devices configured to: receive a request for a first service to perform a particular task involving a second service on behalf of a first user, wherein the first and second services are in communication with the interoperability network and are provided by first and second ones of the independent service providers, respectively; determine whether the first user has provided first set of credentials that defines access information for the second service, the first set of credentials being included among a plurality of sets of credentials stored on one or more storage media accessible through the interoperability network; determine whether the first service provider is authorized to act on behalf of the first user with respect to the second service with reference to one or more of a plurality of permissions stored on the one or more storage media; and where the first user has provided the first set of credentials, and where the first service provider is authorized to act on behalf of the first user with respect to the second service, retrieve the first set of credentials from the one or more storage media, and provide the first set of credentials to the second service for authentication to facilitate performance of the particular task.