Basic Auth for Wordpress.
With this plugin you can protect your WordPress installation with Basic Auth. Basic Auth can use custom password or WordPress users login data.
This plugin protects all dynamically generated content: posts, pages, archives, etc. Basic auth does not protect static content like images, scripts and CSS files.
You can protect: admin area (wp-admin), login page and frontend area.
You can install this plugin like any other WordPress plugin.
You can also use WordPress uploader to upload plugin zip file in menu Plugins -> Add New -> Upload Plugin. Then go directly to point 3.