Gmail Notifier is no longer available for download. Take a look at POP Peeper instead.
Gmail Notifier enables you to check your Gmail account right from the system tray, that is, without having to visit the Gmail webpage. Although this idea seems quite convenient at first sight, the truth is that I found this little app a bit disappointing.
First, you can only check one account, while other similar notifiers such as Gmail Manager have support for multiple Gmail accounts. Also, Gmail Notifier only checks your mailbox when you tell it to do so, as there's no automatic check feature included.
On the upside, the program shows message snippets every time you check email, so you can decide if it's worth opening the browser to read them or not.