Now you can add Google +1 button in your kentico cms site using free webpart. You can download web part from here http://devnet.kentico.com/Marketplace/Web-parts/Google–1-Button-Web-Part.aspx
You can get more details about this web part here
Google +1 Button for your Kentico CMS website
Google +1 button is works works same like Facebook Like button.
Why we need to put Google +1 button in your web site. Here is the nice video about Google +1 button use