Today, I've merged my Gnus notifications module inside Gnus git repository. This way, it will be available for everybody in Emacs 24.2.
This module allows you to be notified via
(the Emacs implementation of the Freedesktop desktop notifications) on new
messages received in Gnus. It can also retrieves contacts photo via
gravatar.el and google-contacts.el to include them in the notification.
To enable it in Emacs > 24.1, you just have to add the following line to your Gnus configuration file:
(add-hook 'gnus-after-getting-new-news-hook 'gnus-notifications)
If you want to download it and use it stand-alone for a previous Emacs version, you can fetch the latest file revision and load it before adding the previously given line.