really simple program, basically like paint. You can easily crop images, resize them (it doesn't fuck up with resizing in the way MSPaint does), do all the really basic stuff. I use it all the time, it's really handy.
Linux users can try Krita too. It's a promising program.
MyPaint is a really good program. Very simple interface and very good brushes. My only concern about it is being unable to define the document resolution. But it can be solved by working bigger than the desired size. I like it. But it's better be used with an image editor like Gimp or Photoshop. It's not an alternative for those. But it's a good Sketchbook Pro or Corel Painter alternative.