There are so many Gmail Notifier is Avialbale on the net you can chack some of them here and here. But if you are a hotmail user and looking for the same, than here is the new app Hotmail Email Notifier. Just put Hotmail.exe into your startup folder. The icon will change to a dude with a red letter if you have unread e-mails. A single click onto the icon opens your Windows Live Hotmail inbox.
Thanks for sharing all these tools