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
notifications-notify (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.