This code is based on the great share: http://forum.unity3d.com/threads/217408-TexturePacker-to-Sprite

And I added the TexturePivotParser, which is a complement of the library, as the TexuturePacker does not support the pivot setting in Unity3d 4.3.
It can be convenient if you need to set some of your frames in the sprite sheet a different pivot point.

One thing to note is that, if you updates the sprite sheet quite often, 
you can add a prefix to the sprites, like "000001_", 
which will help keep your sprites sorted in order in Unity, 
as Unity sort the sprites in side a sprite sheet based on the alphabetic order.


There is a tutorial for this plugin here:
http://www.youtube.com/watch?v=CHQmvC1pqaY

http://www.texturepacker.com/
How to import texture sheets from Texture Packer

TexturePacker settings:
	Data Format:  Unity3D   (or JSON Hashtable, then change extension from .json to .txt so Unity picks it up as a text asset)
	Allow rotation is OK
	Everything else at your discretion
	I suggest keeping the output texture's size as Power of 2.
	
	
Unity process:
	Create a folder in your Assets/ directory for your imported sprites.
	Copy the TXT and Image file (PNG, TGA, etc) into that folder.
	Your paths should look something like:
		Assets/MySprite/MySprite.txt
		Assets/MySprite/MySprite.png
		
	

TexturePivotParser:
	Create a text file in json format containing the pivot data for the frames (name does not matter).
	Put it in the same folder as the sprite sheet.
	Your paths should look something like:
		Assets/MySprite/MySprite.txt
		Assets/MySprite/MySprite.png
		Assets/MySprite/MySpritePivot.txt
	Right click on the pivot file and choose PivotParser/Process for pivot.
	
	format of the pivot file:
		{
		"meta": {
			"image": "MySprite.png",	
		},
		"pivots": {
			"frame_0.png": "bottom left",
			"frame_1.png": "top left",
			"frame_2.png": "top right",
			"frame_3.png": "center"	
		}
		}
	
	Note: 
		The pivot value is the same as what you can set in the Sprite importer, but in lower case.
		However, the custom type is not supported.
			

	

Shaders:
	Transparent Unlit - 
		The default shader for all imported sprite sheets.
	Opaque Unlit - 
		nontransparent tintable shader great for drawing backgrounds that don't need alpha.  Very efficient.
	Vertex Color - 
		Does not have an inspector-tweakable color property.  All colors must be set by altering the colors[] or colors32[] array of a given mesh.  
		Supports both texture alpha and vertex color alpha.