Pinta is like paint.net on Linux but way less polished (it's image editor core started as a port of Paint.net 3.0's when paint.net was still open source, but it's UI was a clone in GTK). I remember being particularly unimpressed when I tried using it for low res image manipulation and discovered the selection "marching ants" and resize handles were rendered at the canvas resolution, for example.

At least for me, Krita has ended up as my paint.net replacement, but it is a step up in complexity from paint.net (and hence two steps up from paint)

For me, MS Paint is really good at pixel art. Pinta seems to be good at it as well. At first glance, Krita looks like more of a painting software, kind of closer to Photoshop than to Paint.