WP Force Like Plugin
WordPress Force Like Plugin allows you to require your visitors to like a facebook page before revealing the page/post contents.
For example if you have a valuable coupon code, download link, a picture or simple text which you would like to show only to fans, this plugin is just for you! Get viral traffic by requesting a like before revealing your content.
Extremely easy to install just like any other Wordpress Plugin.
No API keys required.
WP 3.0 or higher Regular LAMP Server