THe business rule engine in Windows Workflow Foundation is an often overlooked feature that provides powerful business logic processing and rich business rule engine execution semantics.  Some mistakenly believe this functionality can only be used from within a workflow.  I worked with Microsoft recently on a couple of articles to provide developers  hands-on examplesof using rules in .NET applications and in WCF services.   These articles are part of a new series intended to provide small concrete examples of how to use the technology.  You'll find links in the sidebar to other articles on using sequential and state machine workflows.  If you are learning WF , don't forget about our screencast series as well where we have information on WF, WCF and more.