USB Webmail Notifier is a little box with envelope design on the front that lights up in green, blue, or red color when you receive mail. It is powered by USB. It supports all the popular free email providers like Hotmail, Yahoo and Gmail, but also works with Outlook, Outlook Express and POP 3 mail. The price is  $17.  If you are interested in it, you can see the full specifications and buy it here.