Add to a Unity GameObject to easily see the center of a scene in the editor.

DrawGizmoGrid.cs

using UnityEngine;
using System.Collections;

// DrawGizmoGrid.cs
// draws a useful reference grid in the editor in Unity. 
// 09/01/15 - Hayden Scott-Baron
// twitter.com/docky 
// no attribution needed, but please tell me if you like it ^_^

//Forked by gazugafan, 4/14/2018...
//Added individual color options for center, major, and minor grid lines
//Added toggles for vertical and horizontal gridlines--allowing both to be shown at once
//More useful for seeing the exact center of the scene

public class DrawGizmoGrid:MonoBehaviour
{
	// universal grid scale
	public float gridScale = 1f;

	// extents of the grid
	public int minX = -15;
	public int minY = -15;
	public int maxX = 15;
	public int maxY = 15;

	// nudges the whole grid rel
	public Vector3 gridOffset = Vector3.zero;

	// choose a colour for the gizmos
	public bool drawHorizontal = true;
	public bool drawVertical = true;
	public int gizmoMajorLines = 5;
	public Color centerLineColor = new Color(1f, 0f, 0f, 0.5f);
	public Color majorLineColor = new Color(1f, 0.5f, 0f, 0.2f);
	public Color minorLineColor = new Color(1f, 0.5f, 0f, 0.08f);

	// rename + centre the gameobject upon first time dragging the script into the editor. 
	void Reset()
	{
		if (name == "GameObject")
			name = "Center Grid";

		transform.position = Vector3.zero;
	}

	// draw the grid :) 
	void OnDrawGizmos()
	{
		// orient to the gameobject, so you can rotate the grid independently if desired
		Gizmos.matrix = transform.localToWorldMatrix;

		// draw the horizontal lines
		for (int x = minX; x < maxX + 1; x++)
		{
			if (x != 0)
				DrawXLine(x);
		}

		// draw the vertical lines
		for (int y = minY; y < maxY + 1; y++)
		{
			if (y != 0)
				DrawYLine(y);
		}

		DrawXLine(0);
		DrawYLine(0);
	}

	private void DrawXLine(int x)
	{
		// find major lines
		Gizmos.color = (x % gizmoMajorLines == 0 ? majorLineColor : minorLineColor);
		if (x == 0)
			Gizmos.color = centerLineColor;

		Vector3 pos1 = new Vector3(x, minY, 0) * gridScale;
		Vector3 pos2 = new Vector3(x, maxY, 0) * gridScale;

		if (drawVertical)
			Gizmos.DrawLine((gridOffset + pos1), (gridOffset + pos2));

		pos1 = new Vector3(pos1.x, 0, pos1.y);
		pos2 = new Vector3(pos2.x, 0, pos2.y);

		if (drawHorizontal)
			Gizmos.DrawLine((gridOffset + pos1), (gridOffset + pos2));
	}

	private void DrawYLine(int y)
	{
		// find major lines
		Gizmos.color = (y % gizmoMajorLines == 0 ? majorLineColor : minorLineColor);
		if (y == 0)
			Gizmos.color = centerLineColor;

		Vector3 pos1 = new Vector3(minX, y, 0) * gridScale;
		Vector3 pos2 = new Vector3(maxX, y, 0) * gridScale;

		if (drawVertical)
			Gizmos.DrawLine((gridOffset + pos1), (gridOffset + pos2));

		pos1 = new Vector3(pos1.x, 0, pos1.y);
		pos2 = new Vector3(pos2.x, 0, pos2.y);

		if (drawHorizontal)
			Gizmos.DrawLine((gridOffset + pos1), (gridOffset + pos2));
	}
}