Alex Sawallich

This plugins enables the famous Google +1-button in your wordpress-installation. The button is shown beneath 
your posts/pages. With this button you can share the permalink to your post/page in your Google+ profile.
You’ve got the possibility to change the look of the button in the same manner, that you can do by using the 
configurator that Google provides: see <a href="http://www.google.com/webmasters/+1/button/" rel="nofollow ugc">Google plus one</a>.

Google+ One Button

Provides a +1 button from Google+ beneath each post/page.

benjick

Add a Google+ Page button easily with this plugin. The shortcode is <code>[googlepage]</code>
Usage: 
* Smallest icon: <code>[googlepage page=YOUR_PAGE_ID]</code> 
* Medium icon: <code>[googlepage size=32 page=YOUR_PAGE_ID]</code> 
* Biggest icon: <code>[googlepage size=64 page=YOUR_PAGE_ID]</code>
In page=x where x is your Google+ Page number.

Google Plus Page shortcode

This plugin allows you to add links to a Google+ Page with simple shortcode

Google+ One Button vs Google Plus Page shortcode