Philip John

Need to make sure that a site is private for all but the administrators? Just activate this plugin.
There are no settings, no configuration – it just forces anyone that isn’t an admin to go to the login page. They won’t see anything of the site at all. Great for hiding sites really quickly.

Simple Site Lockdown

Provides a really simple mechanism for locking down a site so that it's private to all but logged in admin users.

MythThrazz

The plugin is very simple and it doesn’t come with any changable options.
The only feature it has is ability to detect the AdBlock (tested on Chrome AdBlock extension) and defines the HTML class allowing to control the visibility of 
the elements on your page based on the detection result eg. you can show an element on your page only if AdBlock is detected, or hide it.

Tiny Simple AdBlock Detector

Detects AdBlocking software and allows to display some html based on detection result. Word for AdBlock for sure. Should work for others.

Simple Site Lockdown vs Tiny Simple AdBlock Detector