Photoshot and GIMP are the main two graphic application available for power users or professionals. Those softwares are pretty rich in features. At the same time they are fairly complex to use. It in fact require a training to use.
Those who are interested to experiment with an image, I suggest to use GIMP as a starting point. But I will not reccoment GIMP for a normal users whose only need is make some minor changes in an image..
For normal users, I suggest to use Paint.NET (Windows) or Pinta (Ubuntu/Linux). Both are fairly simple compare to Photoshop/GIMP.
I personally prefer using GIMP because it is free and still extremely powerful with the correct plugins. Photoshop is way too expensive for me to justify the cost when I am not professionally making graphics.