Download the Windows Mobile 6 SDK. Main features added in WM6 includes:

• Simplified Line of Business Application Development and Deployment with integration of .NET Compact Framework 2.0 Service Pack 1 (SP1) and Microsoft SQL Server™ 2005 Compact Edition into the ROM of each device.
• Enhanced emulator with a fake GPS support and a real network simulator
  • FakeGPS allows you to test your location-enabled applications without needing access to a GPS device.
  • A new Cellular Emulator that works in conjunction with the Device Emulator to enable you to test your application's behavior in a variety of cellular scenarios such as incoming phone calls, incoming SMS messages, dropped phone calls, and dropped data connectivity (this deadly miss to Symbian SDK righht now!).
  • The improved Device Emulator also allows you to test your applications across a wider range of device form factors without needing to purchase the device.
• Enhanced User Experience

You will get some more details on this What's New for Developers in Windows Mobile 6 article from Microsoft.

Two SDK are also available right now for download. They both add documentation, sample code, header and library files, emulator images and tools to Visual Studio that let you build applications for Wndows Mobile 6 phones..

These SDK are available for free download from Microsoft Download Center.

Note that due to new and dumb naming convention from Microsoft, the "Standard SDK" targets the smartphones while the "Professional SDK" targets "Pocket PC Phone Edition" family of products.