CameraFacingBillboard http://wiki.unity3d.com/index.php/CameraFacingBillboard

CameraFacing.cs

//	CameraFacing.cs 
//	original by Neil Carter (NCarter)
//	modified by Hayden Scott-Baron (Dock) - http://starfruitgames.com
//  allows specified orientation axis
 
 
using UnityEngine;
using System.Collections;
 
public class CameraFacing : MonoBehaviour
{
	Camera referenceCamera;
 
	public enum Axis {up, down, left, right, forward, back};
	public bool reverseFace = false; 
	public Axis axis = Axis.up; 
 
	// return a direction based upon chosen axis
	public Vector3 GetAxis (Axis refAxis)
	{
		switch (refAxis)
		{
			case Axis.down:
				return Vector3.down; 
			case Axis.forward:
				return Vector3.forward; 
			case Axis.back:
				return Vector3.back; 
			case Axis.left:
				return Vector3.left; 
			case Axis.right:
				return Vector3.right; 
		}
 
		// default is Vector3.up
		return Vector3.up; 		
	}
 
	void  Awake ()
	{
		// if no camera referenced, grab the main camera
		if (!referenceCamera)
			referenceCamera = Camera.main; 
	}
 
	void  Update ()
	{
		// rotates the object relative to the camera
		Vector3 targetPos = transform.position + referenceCamera.transform.rotation * (reverseFace ? Vector3.forward : Vector3.back) ;
		Vector3 targetOrientation = referenceCamera.transform.rotation * GetAxis(axis);
		transform.LookAt (targetPos, targetOrientation);
	}
}

CameraFacingBillboard.cs

using UnityEngine;
using System.Collections;
 
public class CameraFacingBillboard : MonoBehaviour
{
    public Camera m_Camera;
 
    void Update()
    {
        transform.LookAt(transform.position + m_Camera.transform.rotation * Vector3.forward,
            m_Camera.transform.rotation * Vector3.up);
    }
}

CameraFacingBillboard2.cs

//cameraFacingBillboard.cs v02
//by Neil Carter (NCarter)
//modified by Juan Castaneda (juanelo)
//
//added in-between GRP object to perform rotations on
//added auto-find main camera
//added un-initialized state, where script will do nothing
using UnityEngine;
using System.Collections;
 
 
public class CameraFacingBillboard2 : MonoBehaviour
{
 
    public Camera m_Camera;
	public bool amActive =false;
	public bool autoInit =false;
	GameObject myContainer;	
 
	void Awake(){
		if (autoInit == true){
			m_Camera = Camera.main;
			amActive = true;
		}
 
		myContainer = new GameObject();
		myContainer.name = "GRP_"+transform.gameObject.name;
		myContainer.transform.position = transform.position;
		transform.parent = myContainer.transform;
	}
 
 
    void Update(){
        if(amActive==true){
        	myContainer.transform.LookAt(myContainer.transform.position + m_Camera.transform.rotation * Vector3.back, m_Camera.transform.rotation * Vector3.up);
	    }
    }
}