Well, good news, this solution is tested :) I was curious why it didn't work, the logic seemed right. Turns out, for whatever reason, having an Input() in the OnGUI() call is a bad idea. I was getting double-clicks of my escape key. By moving Input() down to Update(), it worked fine. Also note I used GetKeyUp(KeyCode.Escape)

The actual complete (C#) code. Note that this should be in a file called *s_GUI.cs*, to match the Class name:

using UnityEngine;
using System.Collections;

public class s_GUI : MonoBehaviour {
bool quitMenu = false;

    void OnGUI() {
        if (quitMenu == true) {
            if (GUI.Button (new Rect(20, 20, 20, 20), "Quit")) {
                quitMenu = false;
                // application stuff.
            }
        }
    }

    void Update () {
        if (Input.GetKeyUp(KeyCode.Escape)) {
            quitMenu = !quitMenu;
        }
    }
} // end class s_GUI

Here is a more complete example written in JavaScript. Just copy/paste it as it is and assign it to an Empty Object to see this example in action.

var width: int = 256;
var height: int = 256;
var skin: GUISkin;

private var rect: Rect;
private var show: boolean = false;

function Awake() {
    var x = (Screen.width * 0.5) - (width * 0.5);
    var x = (Screen.height * 0.5) - (height * 0.5);
    rect = Rect(x, y, width, height);    
}

function Update() {
    if (Input.GetKeyDown(KeyCode.Escape)) {
        show = !show;
    }    
}

function OnGUI() {
    GUI.skin = skin;    
    if (show) {
        GUILayout.BeginArea(rect);
        GUILayout.Box("Pause Menu");
        if (GUILayout.Button("Continue")) {
            show = false;
        }
        if (GUILayout.Button("Restart")) {
            Application.LoadLevel("Current Level");
        }
        if (GUILayout.Button("Quit")) {
            Application.LoadLevel("Opening Screen");
        }
        GUILayout.EndArea();
    }
}

Now in more details. From the inspector you are able to change the width and height of your menu that will be displayed in the center of the screen (the code for that is in the Awake() function). In order to draw the GUI I'm using the Button and Box functions from GUILayout. For a better user experience you can also create a GUISkin and assign it to the skin variable in the Inspector.

Good luck and have fun!

This is off the top of my head, not tested...

private var quitMenu;

function OnGUI () {

if (Input.GetButtonDown ("Esc")) 
{ 
    if (quitMenu == true) { quitMenu = false; }
    else { quitMenu=true; }

}

if (quitMenu == true)
{
    if (GUI.Button (Rect(10,10,50,50),"Quit"))
    { 
        Application.LoadLevel("Opening Screen");

    }

}

}