Take Recent Notifications.

In a terminal run:
sudo add-apt-repository ppa:jconti/recent-notifications
sudo apt-get update
sudo apt-get install indicator-notifications
Alternatively you could download the package directly on the launchpad-site. For Ubuntu 11.10 Oneiric on an i386-machine choose indicator-notifications_0.1.4-2~ppa~oneiric_i386.deb. (Don't install recent-notifications)
After installing run Alt+F2 unity --replace.
You will find a little mailbox in the panel:

It is possible to align the icon to your desktop theme, i. e. when you use Faenza:
mkdir ~/.icons/Faenza/status/scalable
cp /usr/share/icons/Faenza/status/scalable/user-available-symbolic.svg ~/.icons/Faenza/status/scalable/indicator-notification-unread.svg
cp /usr/share/icons/Faenza/status/scalable/user-away-symbolic.svg ~/.icons/Faenza/status/scalable/indicator-notification-read.svg

To stop it you need to unistall it:
sudo apt-get purge indicator-notifications
Then again run Alt+F2 unity --replace.
To remove the ppa run:
sudo add-apt-repository -r ppa:jconti/recent-notifications