Aaron D. Campbell

Sometimes it’s a private blog, and that’s all there is to it. If all you want 
to do is limit your blog to only registered users, look no further. Simply 
install this plugin, and stop worrying.

Authenticate

Instantly requires that users be logged in to visit your site. Also serves as a good base for expansion. No interface, just activate and go!  Requires …

Takashi Kitajima

This plugin allows only users to authenticate to WordPress.
<ul>
<li>Subscriber can’t see dashboad.</li>
<li>Subscriber can’t see Admin Bar.</li>
<li>Add ‘Disallow: /’ in robots.txt</li>
</ul>
このプラグインはWordPressで認証されたユーザーのみ許可します。
・「購読者」はダッシュボードに入れないようにします。 
・「購読者」には管理バーを表示しません。 
・robots.txt に ‘Disallow: /’ を追加します。

MW Auth

I use this plugin for a test site.
very useful.
Thanks!

Thanks!

This plugin allows only users to authenticate to WordPress.

Authenticate vs MW Auth