The Photoshop plugins from MacPhun are Mac only. I love running their Intensify CK (Creative Kit) plugin on backgrounds. I especially like the Gloomy Day preset. I will present two examples. First, here is an unretouched HDR image:
Here is the image with the Gloomy Day preset applied:
The sky was replaced, as I wanted a sky with more clouds and some yellow and oranges to complement the colors of the flowers in the foreground. Sara was then added for some interest:
Here is a second starting image:
With the Gloomy Day preset applied, we get this:
Some light was added streaming through the window, and Vivienne placed in front:
If you are a Mac user and want to darken and add interesting contrast to a background to make the composited model "pop," then this plugin is a must have.