I'm a regular user of Gmail Notifier, the Google application that works on your system tray and alerts you when you have new Gmail messages on your account... for me it's really useful.
I was thinking how useful could be an application like this when I'm working with my Linux Box, so I've done a little search around the net (impossible that with the Gmail diffusion, someone on the Linux world doesn't think to this...) and I've found KCheckGmail, a KDE system tray application which notifies you of new email messages in a Gmail account.
Really cool! 