I have Paint.net (free) for easy/ quick jobs and I haev GIMP for more detailed jobs. At least that is how it works in theory, In reality I barely know how to work Gimp so i never use it.
I do plan on learning some day. I have it penciled in tenativley around the time hell freezes over.
It is a really go program though. I gave it to a graphic designer friend and she loved it.