ACDSee Photo Flash is a very basic photo optimization tool. It allows you to improve the lighting and contrast of your photos very quickly, and, most importantly, easily. After loading an image in the application, it will be automatically enhanced. Below the image, there is a sliding bar that will increase or decrease brightness and contrast when you move it to the either side. There are two buttons next to the bar. The one on the left lets you see the loaded image in its original state, and the one on the right resets any changes that you may have made to the image.
In most cases, you will get good results with dark images. In my testing, I found that the application's automatic enhancement was enough, but some of the images that I used to test this app required me to move the bar to the right a bit for better results.
If you need to quickly enhance the lighting of dark images, ACDSee Photo Flash could be useful, but you can do the same with iPhoto and you have more control of the process. Also, you are limited to optimizing one photo at the time, which can be really time consuming.