DraggableObject.cs

using UnityEngine;
using System.Collections;

public class DraggableObject : MonoBehaviour
{
    bool dragging = false;
    Plane movePlane;

    void OnMouseDown ()
    {
        dragging = true;
        movePlane = new Plane(-Camera.main.transform.forward,transform.position);
    }

    void Update ()
    {
        if (!dragging || !this.enabled)
            return;

        if (Input.GetMouseButtonUp(0))
            dragging = false;

        Ray camRay = Camera.main.ScreenPointToRay(Input.mousePosition);
        float hitDist;
        if (movePlane.Raycast(camRay,out hitDist))
            transform.position = camRay.GetPoint(hitDist);
    }
}