Creating Custom Application Page ( Code Behind )

Scenario:
You want to write a Custom Application Page for some additional configurations for all of your sites.

MyConfig.ASPX(12/TEMPLATE/Layouts/Client/):

<%@ Assembly Name="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Assembly Name="CustomApplicationPages, Version=1.0.0.0, Culture=neutral, PublicKeyToken=d4e5777b16a5749f" %>

<%@ Page Language="C#" MasterPageFile="~/_layouts/application.master" 
         Inherits="CustomApplicationPages.MyConfig"
         EnableViewState="false" EnableViewStateMac="false" %>

<asp:Content ID="Main" contentplaceholderid="PlaceHolderMain" runat="server">
  <table border="1" cellpadding="1" cellspacing="0" style="font-size:12">
    <tr>
      <td>Site Title:</td>
      <td><asp:Label ID="lblSiteTitle" runat="server" /></td>
    </tr>
    <tr>
      <td>Site ID:</td>
      <td><asp:Label ID="lblSiteID" runat="server" /></td>
    </tr>
  </table>       
</asp:Content>

<asp:Content ID="PageTitle" runat="server"
             contentplaceholderid="PlaceHolderPageTitle" >
 Hello World
</asp:Content>

<asp:Content ID="PageTitleInTitleArea" runat="server"
             contentplaceholderid="PlaceHolderPageTitleInTitleArea" >
 Custom Application Page : 'Hello World' with code behind
</asp:Content>

MyConfig.cs(GAC ,CustomApplicationPages.dll)

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace CustomApplicationPages {

  public class MyConfig : Microsoft.SharePoint.WebControls.LayoutsPageBase {

    // add control fields to match controls tags on .aspx page
    protected Label lblSiteTitle;
    protected Label lblSiteID;

    protected override void OnLoad(EventArgs e) {
      
      // get current site and web
      SPSite siteCollection = this.Site;
      SPWeb site = this.Web;
      
      // program against controls on .aspx page
      lblSiteTitle.Text = site.Title;
      lblSiteID.Text = site.ID.ToString().ToUpper();
    }
  }
}

Usage:
You can access to the page from any web i.e.

 http://localhost/_layouts/client/myconfig.aspx

or you can also write a CustomAction to access the URL