Artefact Animator provides an easy to use framework for procedural time-based animations in Silverlight and WPF. Download the latest version from http://artefactanimator.codeplex.com/.

Inspired by the Flash community and forged out of frustration, Artefact Animator brings to WPF and Silverlight what Tweener has done for Flash. If you’ve ever tried to get into animation in C# you’ll soon realize there are way too many steps to get you where you need to be. The consequences are that developers don’t love to animate UI and designers are limited by what they can animate when working with dynamic content.

via http://www.artefactgroup.com/blog/2010/04/artefact-animator-live-and-open-source-for-silverlight-and-wpf/