The HDRI Shadow Compositor, or "Shadow Compositor" for short, is a node I created that allows its user to quickly and easily composite a realistic looking shadow (and caustics) onto a backplate with little adjusting to the node itself. 

The scene doesn't need to be re-rendered for tuning of the shadow, enabling a faster production for highly realistic renders. You can now use the wonderful HDRIs you've been saving up for so many years... and still get sharp, clean, realistic shadows from them.