A very simple WordPress plugin that redirects all non-admin visitors to the homepage or a page of your choice.
Anyone logged in with admin access can view and edit the entire site as usual.
You can always still access the wp-admin login page.
This may be useful if you are developing a site, but only want visitors to see a landing page.
You can also choose whether 404 pages are sent to the regular 404 page or the home page as well. This could be useful for redirecting Google links.