CLIQ

Clique is a simple addon that enables powerful click-casting and hover-casting on your unit frames and in the 3D game world.

You can bind virtually any mouse or keyboard combination to a spell or macro. In it's normal configuration this enables you to use the bindings over your unit frames in order to cast spells directly on that unit. This allows you to more quickly select both the spell to cast, and the target of the spell without requiring an extra click.