With Gmail becoming so popular, there's now several Gmail notifiers out there promising to deliver mail to your desktop. Gdow is certainly one of the most flexible I've used allowing you to configure exactly how you want your mail delivered.

Once a new message has arrived in your Gmail inbox, you can select how you want Gdow to deliver it to your desktop. The choices are pop-up window or a warning bubble and these can be formatted in Windows XP style or a simple transparent window. The program sits quietly in your system tray and can be easily deactivated by right clicking on it and switching off the notifications.

What I like is that you can configure the notifications as you like them. You can change the background colour, the font used, the width and the maximum height of the notification boxes.
If the message is too big, the message intelligently autoscrolls along your screen. You can even adjust the transparency of the window. It also supports "Gmail for your domain" which is great if you get your website mail delivered to your Gmail.

Overall, this is one of the best notifiers I've used for Gmail with only downside being that the notifications often lag quite some time behind the actual arrival of the mail.