Windows Workflow Foundation is a very cool and useful addition to the .NET Framework.  Interested in learning more about writing custom activities for WF?  Check out Matt Milner's article in the December 2006 issue of MSDN Magazine.  Note, too, that as part of our new Foundations column we've started with the January 2007 issue of MSDN Magazine, you'll be getting a lot more great information on WF from Matt.

-Stephen