Some people use Gimp for pixel art, that one's free. Complex UI and a learning curve on everything, but it's a very comprehensive program. It's a useful program to have around anyway, for adding text, filters, adjusting color balance, and more.
I recently decided to invest in a MacBook Air and I absolutely love it! The only problem is that I'm not sure which program I should use to make art with, especially pixel art.
I've always used an older version of MS Paint and now I feel completely lost. To be honest, I haven't tried to download a Paint program on here or anything but I really was hoping I could get some feedback.
I don't currently use Apple products for art but I've made some research because I wanted to get the iPad Pro and the best art program for iOS seems to be Procreate. I think it has an in-built pixel brush or at least an option to add such a tool.