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:

http://www.raywenderlich.com/32049/texture-packer-tutorial-how-to-create-and-optimize-sprite-sheets-in-cocos2d