Michele recently wrote a lengthy but deliciously thorough article on MSDN (hat tip: Lynn) called Application Deployment Strategies which identifies five core scenarios for using Windows Communication Foundation within a distributed application.  Those scenarios include:

Enterprise web services.   Think secure, interoperable services that may also utilize new-ish WS* standards such as WS-AtomicTransaction and WS-ReliableMessaging.
Web 2.0 services.  […]