Windows Presentation Foundation (WPF), one of the components of .NET Framework 3.0, is the newest presentation technology for the creation of next-generation Windows applications. XAML-based and graphics-oriented, WPF enables developers and designers to create rich and effective user experiences that incorporate UI, media, and documents. Need examples? Applications that are built using WPF include Microsoft Expression Blend, the New York Times Reader, and Yahoo Messenger for Vista. WPF is part of Windows Vista and runs on Windows XP SP2 and Windows Server 2003.
For more WPF goodness, check out the WPF community site, the WPF SDK docs (in particular, the Introduction to Windows Presentation Foundation article), and the WPF SDK team blog.
Samples PM, Windows SDK Team