The controlID of the current hot control.

The hot control is one that is temporarily active. When the user mousedown's on a button, it becomes hot.
No other controls are allowed to respond to mouse events while some other control is hot.
once the user mouseup's, the control sets hotControl to 0 in order to indicate that other controls can now respond to user input.

	// Click on the button to see the id	
	function OnGUI() {
		GUILayout.Button("Press Me!");
		Debug.Log("id: " + GUIUtility.hotControl);
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnGUI() {
        GUILayout.Button("Press Me!");
        Debug.Log("id: " + GUIUtility.hotControl);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnGUI() as void:
		GUILayout.Button('Press Me!')
		Debug.Log(('id: ' + GUIUtility.hotControl))