Google Plus doesn't have #hashtags like Twitter does, but with this extension you can do _scoreTags. 

UPDATE: Google plus now supports hashtags, so this project isn't relevant.

Whenever any post has a _scoreTag in it (a tag with the underscore before it) it will automatically be linked to all recent public posts by other Google+ users that also used that tag. 

Note: tags can appear in comments as well as posts, so you can add _scoreTags to other user's posts.  

Currently only supports Google Chrome. 


UPDATE: This project is now irrelevant. Google plus now supports hashtags. 

This does provide a nice little starting point to create your own chrome extension, so feel free to use it as such.