Sean Fisher

This plugin will hide WordPress Admin (/wp-admin/) when a user isn’t logged in. If a user isn’t logged in and they attempt to access WP Admin directly, they will be unable to and it will return a 404. It can also rename the login URL.
Also, you can add HTTP authentication directly from WP Admin and add custom username/password combinations for the HTTP auth or use the WordPress credentials.
This doesn’t touch any .htaccess files or change the WordPress core files. All the CSS/Images under /wp-admin/ are still accessible, just not the .php ones.
If you enable HTTP authentication, it will add HTTP authentication to the PHP files in /wp-admin/.
To contribute to the development, check out <a href="https://github.com/srtfisher/Lockdown-WPAdmin" rel="nofollow ugc">the GitHub Repository</a>.

Lockdown WP Admin

Lockdown WP Admin conceals the administration and login screen from intruders. It can hide WordPress Admin (/wp-admin/) and and login (/wp-login.

itthinx

The plugin protects a site so that only members can access its pages.
Suitable for site lockdowns, private sites and sites that are under construction.
It allows to redirect visitors to a particular page or to the WordPress login.
Members who are logged in have access to the site. 
Members must belong to the site’s Registered group, or to a particular group as determined specifically for the site.
The plugin is an extension to <a href="https://wordpress.org/plugins/groups/" rel="ugc">Groups</a> which is required.
The redirection settings can be adjusted in Settings > Groups Blog Protect :
Redirection options can be set for the blog – or for each blog in a multisite setup:
<ul>
<li>no redirection</li>
<li>redirect to a specific post, note that with this option the blog is shown to the visitor but only that page can be visited</li>
<li>redirect to the WordPress login</li>
</ul>
The redirect status code can be selected among:
<ul>
<li>Moved Permanently (301)</li>
<li>Found (302)</li>
<li>See other (303)</li>
<li>Temporary Redirect (307)</li>
</ul>
Site administrators can determine which group is used to protect the site:
<ul>
<li>By default, users must belong to the Registered group to be able to access the site, so any registered user who is logged in will have access.</li>
<li>The constant <code>GROUPS_BLOG_PROTECT_GROUP</code> can be set in <code>wp-config.php</code> to indicate the name of the group to which users must belong to be able to access the site.</li>
<li>For multisites, the <code>GROUPS_BLOG_PROTECT_GROUP_n</code> constant can be used to indicate the name of the group required to access a particular site, where <code>n</code> is the blog ID of the site.</li>
</ul>

Groups Blog Protect

Protect access to blogs via group memberships powered by Groups.

Lockdown WP Admin vs Groups Blog Protect