Yahoo Mail Notifier, also known as Y! Mail Notifier will set a small icon in the lower right corner of your browser and will warn you when an e-mail arrives to your inbox.
It's very easy to use, you only have to insert your email address and your password and it will check your account every 1, 2, 3, 5, 10 minutes (you can choose the interval of time) and will warn you if you have a new email.
Finally, you can customize sound, notification alert, status bar and toolbar icon. It also allows you to map different mouse buttons to open Yahoo Mail in different tabs,windows,etc.