Somebody made a shader for this, but it does not wok as nicely as Chrisbob's one:
People can do this in real time too, by using the Set color effect to Black, the a blur, and set the opacity to like 25%
That is what we did with Blender game engine before the stencyl shadow feature was introduced. We smashed a copy of the model, colored it black.
This way the shadow was animated.