Show a holding page or site to non-logged in users while you develop your site.
Can also be used with different domains, eg send 'www.example.com' to the holding site while you access your site normally at an IP or a different domain.
If the site you are working on is part of a multisite, give it more than one domain, make them point to the same site by putting a symbolic link in the /sites folder, and access the site at one domain while sending the other to holding.
Still being tweaked but should be fine to use -- please report any issues you find or suggest improvements.
There is also an htaccess way of doing this which is probably more suitable if the holding site itself runs on a script.