If you are working on a site that is on a live server and has the live url, you could create a page on another site with a link on it that directs to your development site.
Give your client, or whoever you want to see the development site a link to that page. When they click the link they will be able to see the actual live website. Without the link they are redirected to a "under development" page.
var myReferrer = Request.UrlReferrer;
string thisString = "" + myReferrer;
if (thisString == null || thisString == ("www.A-PAGE-ON-ANOTHER-SITE-WITH-A LINK.html") || Session["legit"] == "yes" )
Session["legit"] = "yes";