EventDispatcher.cs

// C#  built-in event system used in Unity3D
//
// Author: Bartek Drozdz
// Reference: http://www.everyday3d.com/blog/index.php/2010/10/04/c-events-and-unity3d/

using UnityEngine;

public class EventDispatcher : MonoBehaviour 
{
    public delegate void EventHandler(GameObject e);
    public event EventHandler MouseOver;
    
    void OnMouseOver () 
    {
        if (MouseOver != null)
            MouseOver (this.gameObject);
    }
}

EventListener.cs

using UnityEngine;

public class EventListener : MonoBehaviour 
{
    private GameObject s;
    // [...]
    s.GetComponent<EventDispatcher>().MouseOver += Listener;
    // [...]
    void Listener(GameObject g) 
    {
       // g is being hovered, do something...
    }
    
    void OnApplicationQuit()
    {
        // unsubscribing
        s.GetComponent<EventDispatcher>().MouseOver -= Listener;        
    }
}