I'm not sure I would describe blender or the GIMP as simple and easy to use. Free, legal, powerful, versatile yes, but not easy to use. If you want to invest a bit of time and learn to use one, GIMP is great. Blender can do amazing things, but is IMO even harder to master.
If you want simple and easy, you probably want something tuned to what you want to do, without all the complex extras you don't need. So the question is what do you want it for? Photo editing? Realistic 3D rendering? Technical drawing? Making logos? There are a great number of free graphics programs about.
for simple things (or complicated things if you're good with it) google sketch up is good for 3D. I also have autocad educational which is free, although you have to out in the name of a school or educational establishment. Its not too difficult to master the basics and there is loads of places available on the web for help as well
In reply to Captain Gear:
Gimp is good and finally now has a proper OSX native gui. Very powerful but I find it frustrating and a bit slow. There are no shortage of online tutorials though which saved me the other week.