Igor emailed me for my thoughts about his SMTP server built with Windows Workflow. A link for the source code is at the bottom of his post. As Igor points out, WF is well suited to solve more than just business problems, and I think his project is an excellent and practical demonstration of building state machines in WF.
Matt Winkler is looking for feedback on refactoring workflows. Personally, I'd like to see some additional renaming support at the property and activity level that can auto-magically patch up declarative activity bindings.
Many moons ago I wrote a post on unit testing custom activities in WF. I've stuck to this approach over the last year, and I'm still looking for something better. A couple new posts on the topic have come up recently. YooT – Unit Testing custom Workflow Activities. Damir Dobric – Unit Testing: How to create activity type from XOML. Andreas Erben's – Unit Testing: Simple approach to Activity testing and improving the XOML loader.