You can do this very easily by making use of the Dropshadow object. Also there's a dropshadow shader that can do this in a similar, albeit not as flexible, way. Being a shader effect it'll also take far more GPU cycles in some situations especially if you have hundreds of objects. Although you could apply dropshadow shader to an entire layer thus reducing the required shader passes to only one that spans the entire screenspace.
This one uses the Dropshadow object; I found it far more intuitive for this: