Microsoft has released a set of components and other tools for the development of client applications in the .NET Framework. The product, code-named Acropolis, was announced at Tech Ed 2007 and is available as a CTP, or community technical preview, from the new WindowsClient.NET page.
Acropolis is part of the '.NET Client Futures' wave of releases, Microsoft's preview of upcoming technologies for Windows client development. With Acropolis, developers can create a set of modules and build them into a business application. These modules can be hosted within the Microsoft Office 2007 suite or as stand-alone applications. Built-in themes are available, but customization using XAML is also possible.
The product caters primarily to Windows Presentation Foundation development. Check this awesome blog on some common Acropolis questions. Acropolis CTP seems like a very finished product even in this preview. It integrates nicely into the Visual Studio "Orcas" preview. There are some really cool controls to explore in the toolbox. These controls include single and multipart navigation managers, components for providing, updating and publishing data, and several UI controls.