gistfile1.txt

using UnityEngine;
using System.Collections;

public class Drag : MonoBehaviour {

	private Vector3 screenPoint;
	private Vector3 offset;

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
	
	}

	void OnMouseDown()
	{
		this.screenPoint = Camera.main.WorldToScreenPoint (transform.position);
		this.offset = transform.position - Camera.main.ScreenToWorldPoint (new Vector3 (Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
	}

	void OnMouseDrag()
	{
		Vector3 Point = new Vector3 (Input.mousePosition.x, Input.mousePosition.y,screenPoint.z);
		Vector3 tPosition = Camera.main.ScreenToWorldPoint (Point) + this.offset;
		transform.position = tPosition;
	}
}