The VIES client allows querying of the EU VAT database system to check the validity of VAT numbers. This is a fairly essential function to businesses within the EU, and avoids having to use the standard browser-based service, by relying on SOAP RPC, which allows access from within any application.

The application is mainly intended to be a learning tool, showing how to create a SOAP connection inside of an app. The application's page (on the Riff Blog) steps through it all to give you a better understanding.