We looked at FIPLAB's MailTab for Gmail back in June. The app ports Gmail's mobile interface to the Mac and anchors it to the menu bar. Since then, the company has come out with similar programs for Facebook and Google+. The latter appealed to me since I've grown to become a fan of the service.
It does well with most basic G+ functions. I could easily read my stream, manage my circles and view my profile. However, posting to it was a hassle as it never fully loaded my circle list. This is a huge drawback. There's also no way to launch a hangout from within the app because of the mobile interface. You need to go through the browser for that.
If you want a basic app for checking your G+ stream without opening your web browser and don't mind putting up with an ad, give this free app a try.