Intro to Windows Workflow Foundation (Part 1 of 7): Workflow in Windows Applications (Level 100)
Intro to Windows Workflow Foundation (Part 2 of 7): Simple Human Workflow Using E-mail (Level 200)
Intro to Windows Workflow Foundation (Part 7 of 7): Developing Rules Driven Workflows (Level 300)
UPDATE: Check our these two great articles by Igor.
Part 1: How to define an SMTP protocol state machine in a Workflow
part 2: Marrying PowerShell into Workflows (Part 1 of 4)