Unflattener is described as 'free and open source Python module and a command line tool (unflatten.py) that helps you make normal maps for 2D sprites and other graphics. You can use the normal maps it generates to implement dynamic lighting in video games' and is an app in the Development category. There are four alternatives to Unflattener for Windows, Linux and Mac. The best alternative is Laigter, which is both free and Open Source. Other great apps like Unflattener are Sprite Lamp (Paid), Sprite Bump (Paid) and SpriteIlluminator (Paid).
Sprite Bump is an advanced 2D Normal Map Generation Tool. It takes in a single 2D image and generates a normal map and optional ambient occlusion map for use in 3D Lighting for realtime Game Engines ( Unreal Engine, Unity etc.
SpriteIlluminator lets you easily generate normal maps for your 2d game and web projects. Render dynamic lighting & other effects and use paint tools to directly draw on the normal map and fine tune the effects.