Rotator.cs

using UnityEngine;
using System.Collections;

public class Rotator : MonoBehaviour {
	
	private Vector3 lastMousePosition = Vector3.zero;
	private bool mousePressed;
	private Vector3 rotation = Vector3.zero;
	private GameObject target = null;
	
	public void Start() {
		target = GameObject.CreatePrimitive(PrimitiveType.Cube);
	}
	
	public void Update () {
		if(Input.GetMouseButtonDown(0)) {
			mousePressed = true;
		} else if(Input.GetMouseButtonUp(0)) {
			mousePressed = false;
			lastMousePosition = Vector3.zero;
		}
		if(mousePressed) {
			Vector3 newPos = Input.mousePosition;
			if(lastMousePosition != Vector3.zero) {
				Rotate(newPos - lastMousePosition);
			}
			lastMousePosition = newPos;
		}
	}
	
	private void Rotate(Vector3 diff) {
		Debug.Log(diff);
		rotation += diff;
		target.transform.rotation = 
			Quaternion.Euler(rotation.y, -rotation.x, rotation.z);
	}
}