UI automation in Silverlight 2 provides accessibility features and can be used for automated testing as well. Silverlight Spy exposes the automation attached dependency properties on all framework elements.
The Silverlight Dynamic Languages SDK enables developers to use dynamic languages running on the Dynamic Language Runtime (DLR) to build Silverlight applications.
Announcing the Silverlight SDK for Microsoft BING, including Microsoft.Bing.Data - a service proxy for the Bing API, and additional reusable view controls: CloudView, TileView, BandCloudView, StackView ...
The Composite Client Application Guidance is intended to make it easier to design and build modular WPF and Silverlight client apps with rich user interfaces and loosely coupled, independently evolveable, UI pieces.
Visual WebGui is another 'Web like Desktop' platform that offers an open source framework to develop applications that work in *both* DHTML/AJAX and Silverlight
Seadragon is the code name for the DeepZoom component of Silverlight. Seadragon AJAX is a script you can use to embed a DeepZoom viewer anywhere you can paste a script snippet.
The DiffPlex project is a combination of a .NET Diffing Library with a Silverlight and HTML diff viewer. It is released open source under the MS-PL license.
XAML Power Toys is a Visual Studio 2008 SP1 Add-In that empowers WPF & Silverlight developers working in the XAML editor. Its Line of Business form generation tools, Grid tools, DataGrid and ListView generation really shorten the XAML page layout time.