See the question and my original answer on StackOverflow

I suggest you have a look at Windows Workflow Foundation (WF). It's part of the .NET Framework.

It uses the concept of Activities. An activity is a general concept, and is also well suited for "connectors" to other systems. Some out-of-the-box activities are provided (.NET Framework 4 Built-In Activity Library)

Others can be written in .NET (Tutorial: Create a Custom WF Activity), you can also find numerous sample implementations on the web (Workflow SendEmail Custom Activity, FTP Activity for Windows Workflow Foundation, MSMQ Workflow Activities, Building and Testing a WCF Web Service Using Workflow Foundation 4.0) or you can also buy already built specific activities from 3rd parties (Xceed Activities, /n software Workflow Activities, etc.)