Here is a nice open source Silverlight project: Live Geometry
* Live Geometry lets you create interactive ruler and compass constructions and experiment with them. It is CAD-like educational software for teachers and students. It helps visualize and solve geometry problems. * The project is written with Silverlight 4 and C# 4.0 (Visual Studio 2010). Visual Studio 2008 is no longer supported. * The core engine is a flexible and extensible framework that allows you to easily add new figure types and features. The project has two front-ends: WPF and Silverlight; they both share the common DynamicGeometry library.