Pyxel Edit.

Pyxel Edit is a pixel art editor designed to make it fun and easy to make tilesets, levels and animations.

Pyxel Edit is a pixel art design application designed especially for working with bricks. Place tiles to form a level, edit them directly to see how they work together, then export your tileset and level dice and load them into your game. The blocks can also be turned and rotated, still being edited and synchronized. This resource was inspired by Pixothello and Cosmigo Pro Motion, but it was a German step.

It also supports making animations and exporting them as sprite sheets or animated GIFs.

Pyxel Edit is integrated into Adobe Air and can be run on Mac and Windows. (supposedly it also works without Linux or Wine)