Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
IcoFX - IcoFX is an all-in-one solution for icon creation, extraction and editing.
IconLover - Icon editor for Windows.
Piskel - Piskel is a website where designers online create sprites or pixel art.
RealWorld Icon Editor - Create Windows icons and Mac OS icons from images and 3D models in an integrated icon maker.
Lospec Pixel Editor - Free online pixel art program. It's easy to use, intuitive and unobtrusive.