I have used http://codeplex.com/simplestatemachine in one production system successfully (an ASP.NET system for handling 350+ different government business license issuance).
A simple state machine is implemented in Boo, so we just expose the definition in a text file which the system administrator can modify. In the editor we do verification to make sure the editing is correct and the workflow is actually functional.
Today I found this, https://github.com/nblumhardt/stateless, which looks interesting.
We switched to an open source alternative simply because our experience with Workflow was not good.