Does anyone know of a quick way to create drop shadows that cast lighter from inside corners and heavier from protrusions?
It is very simple effect created by using Gradient tool applied to a square shape. Shutterstock does not accept Drop-Shadowed Vectors, thus you can be 100% it is not Drop Shadow tool, but Gradient tool.
This one I've created in 30 seconds -
If you want exactly the same view - as you asked Smooth one - you should use Blend tool as well - the proped tutorial I saw in Vectorboom site (has no link, but you can find it easily yourself)
Also, you can play with a gradient applying it to ellipse with different grey stops...
I've find for you the tutorial you can use for inspiration.