Pixen is poised to become the reigning champ of OS X pixel editors. It supports layers, tablet pressure, and animation. But Pixen has a bug that interrupts my creative flow — if I tap a palette color using my Wacom tablet, it always highlights the color, but usually fails to apply the color. What the…?!
Fortunately, the developers made the source code available. I had almost zero Cocoa experience before opening the source, yet the code was well-organized, and I was able to fix the bug!