A blogger's best friend - Riddle makes it easy to quickly create and embed quizzes, lists and polls on your blog in as little as 60 seconds. “
Like Gate allows you to reveal some hidden/secret content when the user likes the article. Therefore increasing the likeness of your articles.
This is a "Share to Download" plugin, and works for Facebook, Google+ and Twitter.
Facebook Like Lock for WordPress. Locks your content and user can unlock by clicking on like button.
Share your viral Content and get traffic to your website.