RotateCamera.cs

using UnityEngine;
using System.Collections;

public class RotateCamera : MonoBehaviour {

    public float speed = 1f;

    // Use this for initialization
    void Start () {

    }
    
    // Update is called once per frame
    void Update () {
        if (( Input.GetMouseButton (0) || Input.GetMouseButton(1) || Input.GetMouseButton(2)) && Input.GetKey(KeyCode.LeftControl)) {
                transform.localRotation = transform.localRotation * Quaternion.Euler (-Input.GetAxis ("Mouse Y") * speed, Input.GetAxis ("Mouse X") * speed, 0);
                //transform.RotateAround( Vector3.zero, Vector3.up, -Input.GetAxis("Mouse X") * speed );
                //transform.RotateAround( Vector3.zero, Vector3.right, -Input.GetAxis("Mouse Y") * speed );
        }
    }
}