For starters, it supports multiple Google Apps accounts, which is great because I use two.  Next, it is FAST.  It polls the Gmail servers every 15 seconds, so the notifications come in nearly instantaneously.  But my favourite feature by far is how much time it saves me managing mail.  When new mail arrives, Google Mail Checker Plus will show you a list of unread messages, along with several buttons:

Right from the notification interface, you’re able to delete the message, mark it as spam, archive it or just mark it as read.  If you’re dealing with large batches of e-mail regularly, this is an enormous time saver.

If you’re a Chrome / Gmail user, definitely check it out.  If you’re a Gmail user but haven’t switched to Chrome, I’d recommend you give it a try for this extension alone. :)