I like the Silverlight animation system. You just define a storyboard somewhere in your code and in that storyboard you describe how you want a property to change over time. Now this is nice and all, but if you want cool ease-in and ease-out animations you'll have to do some very complex Spline-based composition. This gets even harder if you want to reuse those behaviours for different properties or targets.

Koen Zwikstra did some time on the Tweener library targeted at ActionScript 2 and 3 and came up with a Silverlight port. It includes full source code and some tutorials and a demonstration. The library contains most of the tweening animations you can come up with. Very reusable and surpringly little coding.