ARToolkit, a library of tools for creating augmented reality applications is ported to Silverlight, called “SLARToolkit.” Its features include direct Support for Silverlight's CaptureSource, multiple marker detection, simple black square markers, real-time performance and more. It also uses Matrix3DEx library. While above feature list may mean something to app developers, what it means for consumers is that we’ll soon start seeing augmented reality apps developed using Silverlight. These apps will be able to take advantage of your computer’s webcam to launch “augmented reality” apps – that’s, those that use technologies like computer vision and object recognition, for e.g. There’re already two sample apps available on Codeplex, one being a 3D game called Balder and other a sample that uses Silverlight's 3D projection capabilities. To run these samples, you’ll need a webcam and at least Silverlight 4 runtime.
More info: Documentation