Max OS X 10.8 (“Mountain Lion”—boy do I dislike those cat nicknames) added a new Notification Center feature. It was definitely “inspired” by the excellent Growl notification system, and in my opinion not as good.
However, neither notification system is great for Apple Mail. That’s where Herald comes in.
Herald offers a stylish pop-up bezel window that gives you not only a much better preview of the incoming messages, but via the icons on the bottom of the window, a way to performa sctions on the messages like Delete, Mark as Junk, Reply, View in Mail, Mark As Read, and more. Those are even customizable as to which ones appear.
You also have detailed control over which mailboxes on your Mac will trigger the window, as well as the look of the window. All to an amazing degree of granularity.
There’s tons more info on the developer’s web page for the plugin. If any of this sounds interesting to you, trust me you will love this. And even better, it’s 100% free. But if you do like and use Herald, I suggest you donate a few bucks to the developer as I have. It’s just been updated for 10.8 Mountain Lion compatibility as well.