Texture Packer is a wonderful program to create Sprite Sheet with in seconds for different data format like, cocos2d & Unity3D.
Its not a free program but Texture packer's demo version works pretty well too.If you are a 2d animator & creating PNGs of your animations,then its very easy way to maintain all your PNGs in a single Sprite Sheet.
Some advantages of Texture Packer are:
Very easy to use: You have your sprite sheets created with in seconds. save waste your time dealing with texture positioning.
Optimize your game: The app automatically creates textures in power of two, using all the available space, this means you have more assets in a single sheet.
Works with almost any game engine(as I mentioned it in very first line),i.e. Cocos2D and Cocos2D-X, Lib-GDX, AndEngine, Unity3D or just JSON or plain XML, among many others.
For How to get start with Texture Packer you should follow this link given below: