For extra security you can integrate it with Gnome-Keyring password manager. It also supports audio notifications and lets you set intervals for downloading/checking new mails + the GMailWatcher fully supports multiple g-mail accounts as well.
So if you use Ubuntu 11.04 you can easily install GmailWatcher using the below command.
sudo add-apt-repository ppa:loneowais/ppa
sudo apt-get update
sudo apt-get install gmailwatcher gmailwatcher-themes-extra
|Easily configure via notification menu...|
This is developed by Owais Lone. Anyhow, you can easily add extra themes to it by adding simple CSS/HTML codes which can be acquired from this launchpad page if you'd like.