Monitor your Google email and calendar from the Mac menu bar. Google Notifier for Mac gives you a one-click overview of incoming messages and upcoming appointments, and alerts you about both.
When we told you about the official Gmail notifier for Windows, we mistakenly told you there wasn't a Mac version available. Well, there is. It's not the prettiest notifier out there, but it works.
If you've forgotten Google even makes an official notifier, you're not alone – the search engine giant hasn't updated the application since 2010. This means newer Mac features like the notification center are sadly overlooked. Still, it's a solid way to keep up with your email and be reminded about appointments. Here's how it looks, in case you've forgotten.
What To Expect
Start up Google Notifier, then log into your account. If you use 2-step verification your password won't work – set up an application specific password instead.
Once you're signed in you'll see two new icons in your Mac's menubar: one for Gmail and another for Calendar. Click the envelope to see a list of unread messages:

You know how this works: you can click any message to read it now, or use the compose and inbox options to write a message or read your email. The app directs you to your browser, but you can set an email client for composing if you prefer.

You can also change the notification sound and configure whether an unread count shows up in your menubar.
The calendar icon functions similarly, showing you an overview of upcoming events.

There's no mini-calendar to look at or anything, but you can click through to see the calendar in your web browser. Perhaps my favorite feature is the ability to quickly add an item to any calendar connected to your Google account:

There are limited calendar settings, but you can add an upcoming events count if you want:

Overall it's a very simple app that gives you a way to oversee both your mail and your calendar.
Alternatives
So, is this the best tool out there for the job? The icons are pretty ugly, but beyond that it's basically functional. But how do other apps compare?
Well, if you'd like notifications along with full menubar control of your email, check out MailTab for Gmail. It notifies you, but also puts the mobile version of Gmail a click away:

It's great for the email-obsessive, because you can stem the flow of email in a couple of clicks. Surprisingly, keyboard shortcuts work within the app.
If MailTab is too much for you there's also the very simple Gmail Notifr, which is completely open source with support for Growl notifications. It's similar in functionality to Google's offering, but to me is a more attractive option. Check it out if you want something simpler.
Neither of these apps support the calendar alongside email, which is too bad because I love this functionality – if only because Google Calendar running through the stock Mac Calendar app seems to be a recipe for perpetual error messages. Sure, it's not hard to set up the OS X Calendar app to sync with Google Calendar, but everytime I launch Calendar I'm told something is going wrong. It's annoying.
Having said that, Apple's default Calendar app is a pretty strong alternative to Google's offering. For one thing, the integration with the Mountain Lion's Notification Center is fantastic – especially when compared to Google Notifer's ugly notifications. The calendar integration may or may not be a killer feature for you.
Conclusion

Google: step up. This tool could be fantastic with just a few updates, including support for the Mountain Lion notifications center without the need for Growl. And the menubar icons don't match the Mac look at all – give them a monoscale refresh, with proper sizing, and Gmail addicts will thank you for it.
What's your favorite tool for quickly checking your Gmail and Google Calendar using your Mac? Teach me, and everyone else reading this, what you know in the comments.