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)