piskelapp.com
Piskel is a free, open-source web-based editor for spriting and pixel art. It creates pixel art, game sprites and animated GIFs, and is best used in a desktop browser in landscape orientation. The tools include a live animation preview and export to GIF or PNG, including spritesheets. A separate child-friendly version, Piskel for Kids, is available. Free desktop and offline applications exist for Windows, OSX and Linux.