Soflyy

<h4>Market your content virally on social networks by forcing users to perform a social action before accessing your protected content.</h4>
Check out the 2 minute demo video on YouTube: http://www.youtube.com/watch?v=aheR3_ROt94
Like To Keep Reading is a WordPress plugin that forces visitors to Like you on Facebook to access content on your website.
You can protect entire pages or posts, or just sections of pages or posts (our favorite) – i.e. your visitors can read the first two paragraphs and then must click the Like button to keep reading.
You can force visitors to Like your website homepage, any sub-page/URL on your website, or your Facebook page.
Like To Keep Reading also has optional Google+ and Twitter integration – so instead of Facebook Likes you can get Google +1s and Tweets from your visitors in exchange for access to your content.

Like To Keep Reading

Get more Facebook Likes, Google +1s, and Tweets by forcing users to perform a social action before accessing the rest of your content.

marcucci

This is a very simple plugin that will check to see if a visitor is logged in before allowed your WordPress site to be displayed. 
If they are not it will display the login page of your site. It will also add text on the login page to explain that the site 
required all visitors to login.

Password Protect Plugin for WordPress

Force all visitors to login before accessing your wordpress site and add a message on the login screen explaining that login is required.

Like To Keep Reading vs Password Protect Plugin for WordPress