Behaviors in WPF and Silverlight are one of the features that were introduced with Blend 3, and unlike attached behaviors, they benefit from full designer support in Blend 3, thus providing a much higher level of accessibility to developers and designers alike.

Laurent Bugnion has posted a great tutorial where he walks you through the various aspects of a magnifying glass behavior, and Jeremiah Morrill generates eye candy through a behavior that adds glass effects to your visuals.

Last but not least, the Expression team created a new section on the Expression Gallery that is dedicated to behaviors. I sure like shiny new toys 😉