Paint.NET for Mac for pixel by pixel art creationg Does anyone know any Mac alternative that has the exact features that Paint.NET has when it comes to creating pixel art, and free? 

With Paint.NET I'm able to select whatever pixels in the grid to drag them, or change their colors all at once, or delete them, or to give the mirror/reflect (horizontally/vertically) effect.
Though it is more of a Photoshop clone by its number of features, have you considered The Gimp? I don't know if it is going to fit your exact requirement, but it does have an excessive number of features including a magnified editing mode and color substitution.

Otherwise, have you tried Paint.NET via Wine? It seems that only the v3.5.x version might get somewhere, but with limitations.