Singleton implementation from the 'Unite 2013 - Internal Unity Tips and Tricks' talk (http://www.youtube.com/watch?v=Ozc_hXzp_KU)

UnitySingleton.cs

using UnityEngine;

public class Example : MonoBehaviour
{
    public static Example Singleton;

    void OnEnable()
    {
        if (Singleton == null)
            Singleton = this;
    }
}