Mac OS X: Mailbox is a free app that brings major email functionality to the Mac menu bar by using Gmail's mobile webapp.
Shoehorning the entire Gmail mobile webapp into the menu bar may sound strange, but it works out beautifully. Gmail's mobile web interface was built for very fast, very efficient usage, and it seems to translate well when you've got a tiny icon at the top of your screen representing your entire email account.
It also means that the whole thing is fast (really fast), and that it's secure, since it's basically just a tiny browser window that's allowing you to log into the webapp. The downside to this is that it falls short when it comes to things like notifications, so you'll still have to rely on the official Google Notifier or the chimes from your mobile device to keep on top of that.