Pyxel Edit: a Pixel Art Tool

Take a look at a pixel art drawing application especially designed for working with tiles called Pyxel Edit. The tool lets you place tiles to form a level, edit them directly to see how they all work together, then export your tileset and the level data, and load it into your game. What is more, tiles can even be flipped and rotated, still being editable and synced. 

Pixel Edit also supports making animations, and exporting them as sprite sheets or animated GIFs. Check out some of the features below:

• Tile references with transformations. Draw in one tile and they all update instantly. Tile instances can be rotated and flipped, still referencing the same tile data.
• Animations. Make animations easily and export them as sprite sheets or animated GIFs. Onion skinning helps with aligning things between frames.
• Tileset importing. Pyxel Edit can import images of tilesets or mockups and identify all the unique tiles automatically, allowing you to edit and rearrange old tilesets or doing edits of mockups easily.
• Tilemap exporting. Export your canvas tilemap in XML, JSON or plain text format for super quick game prototyping. You can also repeat the last export operation with a keypress, to quickly re-export for small changes.
• Intuitive UI. Designed to feel familiar to other graphics applications, Pyxel Edit is really easy to learn.

Here are some tutorials by Christopher Yabsley to help you get started:

You can find other tutorials and get the tool here.