Pixen is an open-source graphics editor for Mac OS X specifically designed for pixel artists. To that end, it has a number of unique features, such as realtime tiled editing (for those pesky grass tiles), powerful palette control, an intuitive preview window, and so on. It was coded by the Open Sword Group: Joe Osborn, Ian Henderson, and myself.
Features:
Elegant layer support (the only pixel editor to my knowledge with this)
Laws concerning the use of this software vary from country to country. We do not encourage or condone the use of this program if it is in violation of these laws.