Aseprite is an open source program to create animated sprites.

Its main features are:
* Sprites are composed by layers & frames (as separated concepts).
* Supported color modes: RGBA, Indexed (palettes up to 256 colors),
  and Grayscale.
* Load/save sequence of PNG files and GIF animations
  (and FLC, FLI, JPG, BMP, PCX, TGA).
* Export/import animations to/from Sprite Sheets.
* Tiled drawing mode, useful to draw patterns and textures.
* Undo/Redo for every operation.
* Real-time animation preview.
* Multiple editors support.
* Pixel-art specific tools like filled Contour, Polygon, Shading mode, etc.
* Onion skinning

WWW: http://www.aseprite.org/