Event.mousePosition  

// Prints the current mouse position
function OnGUI () {
var e : Event = Event.current;
Debug.Log(e.mousePosition);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void OnGUI() {
Event e = Event.current;
Debug.Log(e.mousePosition);
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def OnGUI():
e as Event = Event.current
Debug.Log(e.mousePosition)