I majorly heart the Growl. I have it hooked up with the Gmail Notifier to give me a pretty little popup when I get new emails.
Actually, the popups are a lot like Mail.Appetizer (http://www.bronsonbeta.com/mailappetizer) for Mail.app, which is what I use at work. But for my personal email I'd rather just keep it all in the browser interface than importing to Mail. That's where Growl makes my day.