http://fortytwopixels.net/post/21340137996/pixel-art-on-macs
Good pixel art applications on macs are pretty hard to come
by, as most have been abandoned by their developers,
lacking
essential features, full of bugs, overly complicated, or
expensive (*cough* Adobe products).
I have recently rediscovered (it was buggy before, but it’s
better now) an application that actually works, and has all the
usual features (layers, alpha channels, even frame-by-frame
animations, and exporting to GIFs, PNGs, etc.) What’s even
better about this is that it’s free!
Pixen is an awesome pixel art application for Macs (10.6+
, but older versions can be found). It’s still in beta, but having
used it for some time, I can tell you that there are hardly any
bugs at all. It’s a very stable and lightweight app, which
means you can use it on the go without it taking up your
RAM, or continuously charging your laptop."