Webmail notifier can be a great tool for you if you send or receive a lot of emails using free email services like Yahoo Gmail etc… If you use Firefox Webmail notifier is really a must have addon for people like you.
Even though most of these free web-mail services now have Auto refresh feature but still you have to keep the window open for that and it can be distracting sometimes.
With Webmail notifier your webmail service will virtually act like outlook email, and you will get an alert whenever a new mail arrives. You will not need to check mail again and again.