Windows Azure .NET Services provides powerful capabilities to connect disparate endpoints across and through the firewalls in place at multiple organizations.
It also provides a general claims-based authentication service in the cloud that can be used to protect REST based cloud storage and other resources.
These are fairly complex components to understand but this blog post by David Aiken of the Azure Evangelism team links to a short video developed by another member of the Azure evangelism team, Nigel Wattling, that explains it very well.
It is worth a look. Highly recommended.