NullifyNetwork

There's an article on MSDN about how to host the Windows Workflow Foundation design surface (which is a redistributable).

The example code is really complete and worth a look at, it's almost an out-of-the-box copy of just the workflow designer from Visual Studio.

If your end user requires more flexibility than most you can offer them drag-and-drop customisation of particular processes in your system (like creating a new customer could be made to go off and send details to a webservice without you, the developer; needing to get involved).