So far Nigel Sampson has covered building a Trigger "Double Click Trigger" and a Behavior "Mouse Wheel Behavior" so all that's left are Actions and he concentrates on the subject in this post.
Actions I believe are probably the most useful of the three sorts of Behaviors in Silverlight, the sheer amount of variation possible when composing these with events provides a lot of richfunctionality to the Blend user. The real power comes from having all this functionality at your drag and drop disposal, by quickly wiring this all together it leaves you more time for true functionality of your RIA.