Mark Monster has a detailed post on one of the new features that Silverlight 3 introduced - Local Messaging.
This feature supports communication between different Silverlight applications that are running on the same client. This is particular useful in areas like Sharepoint where you offer different parts to be positioned at all places on the screen. It’s possible to even communicate between two Slverlight applications running on different domains. For example one app is running on maindomain.com and the second app is running on maindomain.nl or even more different.