Usually I rename web.config to web.config.bak while I make major changes to a web site. This causes an error page to anyone hoping to visit the site that I am modifying. In ASP.NET 2.0 you can take your web site offline and redirect traffic to a page that you can control its content (i.e. not an ASP.NET error page) simply by adding a web page called App_Offline.htm.
Thanks to Erik Porter's Blog for pointing this out.