Is good old Gimp not an option here? I am by no means a pixel artist and just occasionally need the tools of one but it supports working with a constrained palette, exports and imports most formats that matter, has gridlines if you want them and can kick gradients/blended colours in the head if needs be (or you can at least tell it to not use them), does layers, does animations and is free which is always nice. Normally I would not have suggested it but if you were holding up paint.net as a viable candidate then this would be too.
Alternatively you have things like http://www.aseprite.org/older-versions/ (the program is open source but adopted the paywall approach for newer versions). I can not say I have bothered to try such things on a mac though.
Yeah, I am talking about pixel artist so I will go ahead with Pinta or Paint.net with wineskin wrap. It will be much easier since I do not see anybody here experienced pixel artist for NES/SNES with MAC OS X. If anybody does, please drop a line and it will be grateful to me.