By Unknown

Installing and Using Pantheon-Notify

Pantheon-notify is an elementary-themed notification system.
To install pantheon-notify, install via the terminal:
sudo apt-get install pantheon-notify
To activate, you must override the current notification system. Again, in the terminal:
sudo scratch-text-editor /usr/share/dbus-1/services/org.freedesktop.Notifications.service
Change the Exec line to: Exec = /usr/bin/pantheon-notify.

Save, reboot, and enjoy.

Volume changes will also appear different.
Simply hover over the notification to pull up the option to close the popup.
Thanks to Trung for the tip.

[Update: April 3, 2013]
To revert to the original notification system (notify-osd), open the Notifications.service file again. This time change the Exec line back to Exec = /usr/lib/notify-osd/notify-osd. Reboot.