Notify OSD presents on-screen-display notifications as ephemeral overlays, which
can be clicked through so they don't block your work. It queues notifications,
to prevent them from flooding your screen. And as well as handling standard
notification updates, Notify OSD introduces the idea of appending - allowing
notifications to grow over time, for example in the case of instant messages
from a particular person.

WWW: https://launchpad.net/notify-osd