Demonstration of a screen-centered reticle in Unity

Reticle.cs

using UnityEngine;
using System.Collections;

public class Reticle : MonoBehaviour
{
	public Texture2D reticle;

	void Awake ()
	{
		// Set up reticle
		GameObject go = new GameObject ("Reticle");
		go.transform.position = new Vector3 (0.5f, 0.5f, 0);
		go.transform.localScale = new Vector3(0, 0, 1);
		GUITexture tex = go.AddComponent<GUITexture> ();
		tex.texture = reticle;
		Rect ins = new Rect (-(reticle.width / 2), -(reticle.height / 2), reticle.width, reticle.height);
		tex.pixelInset = ins;
	}
}