You can setup up to 5 different gmail accounts and it will manage all of them automatically and simultaneously. If you have a new email, it will notify you with a sound.
If you access the configuration panel of Gmail Notifier you can setup if the program will start in Windows startup or enable/disable audio notifications.
It is very useful, because from now on you won't have to access Gmail to read your messages and manage them.
Remember that the program only supports Gmail accounts and they must have IMAP server support enabled.