As found on Windows Mobile Team blog entry that points to Nauman Leghari's blog which lists plenty of great SmartPhone Programming articles grouped nicely:  

• Introduction to the .NET Compact Framework
• The Smartphone User Interface
• Smartphone Controls
• Graphics
• Files and Directories
• Mobile Web Services
• Working with Unmanaged Code
• Deploying Smartphone Application
• Interoperability
• Game Programming with Smartphone
• Advanced Topics: Security, Debugging, Performance and such 

I recommend you bookmark this one if you ever intend on building an app or two for Windows Mobile.