Windows/Mac/Linux (Adobe AIR): Email notification application [b2]Gmail Notifier tells you when you have a new email message with slick animations and a full message preview.
Once you've installed the application and added your Gmail account, you'll see a new animated message that shows up in the bottom right corner whenever an email comes in. The settings panel doesn't give too much in the way of customisations, simply letting you assign the frequency that mail is checked, but you can also choose between a number of different animations — and even grab some new ones online.
If you're in the market for a slick notification application for a single Gmail account, [b2]Gmail Notifier is probably an application you're going to want to try. It's free, requires the Adobe AIR platform, and works on Mac, Windows and Linux.