Windows 7

General References

Windows Developer Center
Windows 7 Training Kit
Windows API Code Pack
Windows 7 SDK
Windows 7 Application Compatibility Cookbook
ISV Front Runner program
Channel 9 (including video tutorials!)

XP Mode

XP Mode/Virtual PC site
Intel Virtualization
AMD Virtualization

Taskbar

Microsoft UX Guidelines
XP2Win7 Application
Introducing the Taskbar APIs (MSDN Magazine)

Federated Search

OpenSearch
Federated Search (MSDN)
Creating and OpenSearch Description File (MSDN)

Extended Linguistic Services

Extended Linguistic Services (MSDN)
Unicode Consortium

Direct2D/DirectWrite

Introducing Direct2D (MSDN Magazine)
DirectX Developer Center
DirectX Developer Blog

Sensor and Location API

Windows Sensor and Location Platform
Freescale JM Badge Board
Windows Sensors and Location Platforms
Sensor API (MSDN)
Location API (MSDN)

MultiTouch