Google recently added Content Recommendations feature on +1 button. As per Content Recommendations feature when you hover your mouse cursor on a +1 button, recommendations bar opens and shows other related content from that site even if you are not signed in to Google+.
Previously recommendations feature only available to Developer Preview Group users. After a few weeks Google rolling out recommendations feature for all users across the web.
Web giant says
After a few productive weeks in platform preview, we're rolling out +1 recommendations to all users today. Thanks for your feedback at Google I/O, in the discussion forums, and on our Google+ page. We're always eager to hear from you -- so keep it coming.
Recommendations feature rolling out gradually across world. So be patient one or two days recommendations feature will be added automatically.