Behaviors, Triggers and Actions released with Blend 3 allow for developers to wrap up logic into objects that can instantly add interactivity and functionality to an application just by dragging and dropping. This encapsulation is great for reuse along
with empowering designers.
Jeff Kelly, one of the developers behind Behaviors has 3 posts covering
an Introduction to the API
API Details and Constraining Types
Advanced Behavior Features
. Michael Washington has
a tutorial on creating a very simple Behavior
. And don't miss the
Expression Blend Behaviors Samples
library on CodePlex which offers a collection of extra Behaviors to use, along with source code.