Driven shape keys and control set-ups are well known in animation and a vital tool for any animator dealing with something as simple as eye movements to more complex facial expressions. This type of system has been available in Blender for a while and has been used to great effect by advanced users of Blender (see the Ludwig rig).
Now the Blender User Manual has instructions on how to implement your own Driven Shape Keys and control set-ups. The lesson is fairly short, demonstrating that something so easy to implement can grant a user extensive control over an animation.
