Matt Burnett

The Require Login Plugin allows you to easily restrict access to your WordPress site to only users with login credentials, Providing functionality perfect for intranets or restricted development sites.
Below are some of its features:
<ul>
<li>Require users to log in</li>
<li>Customise login page</li>
<li>Redirect users after logging in</li>
<li>Custom login logo support</li>
<li>Disable password reset and “back” link</li>
</ul>

YumJam Require Login

Disable the public site functionality, always prompt for a login before showing any content, make WordPress and intranet style site

jaffe75

If you are not logged into the site it automatically redirects you to the login screen. 
All you have to do is activate the plugin to turn it on. Deactivate to turn it off. 
After some testing this plugin seems to work fine with multisite

JW Forced Login

That's really all that matters isn't it?



YumJam Require Login vs JW Forced Login