Mac OS X only: Mac users who want notifications of new Gmail messages have had the official Google Notifier for quite some time, but it can only monitor one account. Free menu bar application Notify monitors multiple Gmail accounts and then some.
Notify sits in your menubar and displays Growl notifications whenever a new message hits your inbox. You can also click the menubar item at any time to take a quick look at message previews. Notify separates each account into tabs, so you can quickly switch between which account you're looking at. If you want to read a message in Gmail, just double-click the preview. The app's biggest weakness—at the moment at least—is that it doesn't currently support Google Apps account — but it will in the future.