GMail Notifier is a small, simple and fast extension to show the number of unread mails in your Gmail-Account(s) in front of a button and the titles of the messages in the button-menu. Furthermore this extension gives you the option to open mail-links (mailto:) directly with GMail.
By now this extension isn't supported by its author (http://code.google.com/p/google-mail-oex/), so some problems happen according to changes of GMail pages format. I've copied this to make my own workarounds and sharing them with someone who has those problems too.