HDRI Splitter - Control sky background and sky illumination independently. Make sure you load this blend with the UI intact to get a fast example with instructions.
Does your HDRI lighting ever look wrong, but adjusting the strength doesn't help? This is a Node group that takes two inputs: the sky background image and the sky illumination image. This node will act like a standard HDRI background setup if the same background image is attached to both inputs. One example use: split off a copy of your background image, lower the brightness, then attach to the illumination input of the node. Now you can control how much your set is lighted without changing how the sky looks (or vice-versa). New version 6-13-2015: Glossy reflections of the background that do not match the actual background look weird. Now a control called 'Glossy Blend' will allow control of how much the glossy reflections will show the 'Illumination' input. Most cases will leave this at zero. Perfect example: when the ocean and the sky meet at the horizon, they almost always look exactly the same because at a distance the water acts like a perfect mirror.