Pixelformer is a complete icon tool with which you can edit any existent icon or create your own from scratch.
The program features a quite dark interface that doesn't really suit it, but you can't customize it in any way. What really surprised me was the fact that Pixelformer supports layers (something you don't usually find in small editors) and also the possibility to create icons in just about any size from 16x16 to 256x256. What's more, you can create icons from any other image by importing the file into Pixelformer and selecting the area you want.
On the downside, Pixelformer has also some drawbacks. The first one is a rather scarce set of tools, among which I missed the ability to crop and a more powerful zoom tool. There are no special effects or filters to apply to your icons, nor a configuration menu to customize the program's appearance and behavior.
Pixelformer is a great pixel editor with support for layers and many other interesting features, though it lacks some more advanced tools.