I use Webmail Notifier 2.7.6
It has a ton of nice and easy to use option. The button it has doesnt look out of place on the tool bar. Some of the options range from the time frame you set when it checks your mail box. You can set the priority which mail boxes to be checked first. By just one click it takes you to your mail box no matter what mail service you use.
It works with everything gmail, yahoo mail, hotmail, aol and many more.
I have been using it for some time and have no complains what so every.