DebugConsole.cs

using UnityEngine;
using System.Collections;

public class DebugConsole : MonoBehaviour
{

    private static string debugText = "Hello, world.\n";
    private static bool consoleOn = false;
    private static Vector2 scrollPosition;

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Tab))
        {
            consoleOn = !consoleOn;
        }
    }

    public void Log(string text)
    {
        debugText = text + "\n" + debugText;

        const int max = 1024;
        if (debugText.Length > max)
        {
            debugText = debugText.Substring(0, max);
        }
    }

    void OnGUI()
    {

        if (consoleOn)
        {
            scrollPosition = GUILayout.BeginScrollView (
                scrollPosition, GUILayout.Width (400), GUILayout.Height (100));

            GUILayout.Label (debugText);

            GUILayout.EndScrollView ();

        }
    }
}