I have been using the Nik software Color Efex Pro 3.0 plug-in for Aperture for a couple of months. It has become an integral part of my workflow on many images. I will be showing a couple of adjustments to an image that I feel are valuable and very easily applied with this software.
I enjoy shooting large agricultural equipment in the working environment so that is what I will use for the demonstration of this program. My favorite equipment company for images is John Deere because the bright green and yellow colors always stand out so well against the landscape or sky. The first image below is the original image as it was captured.

Combine at Work 1 by Gary Hamburgh - All Rights Reserved
In the 2nd image I added a polarizing filter that is available in the Nik software. This filter has several adjustments available. They are set to the following defaults:
Rotate – 90%
Strength – 100% (60%) In parentheses it shows the opacity that I set for this image.
These adjustments with this filter give a little more pop to the image and also takes away some of the glare.

Combine at Work 2 by Gary Hamburgh - All Rights Reserved
In the 3rd image the tonal contrast filter was applied. The adjustments that are available are shown with their defaults and my adjustment in parentheses.
Highlight contrast – 30 (15)
Midtone contrast – 30 (15)
Shadows contrast – 30 (20)

Combine at Work 3 by Gary Hamburgh - All Rights Reserved
I feel this filter gives a little more definition to the image. Each of these adjustments was slight but each one helps to enhance the image as I had seen it. I hope these adjustments are clearly demonstrated event though the images are small.
This software program has many other filters which are easy to apply and the opacity can also be adjusted. Another important feature is that you can apply multiple filters as I demonstrated to the same image. I hope you have found this information to be useful and consider adding this useful program to your workflow.