To time many applications show passive popups informing you about various events like new email(KMAIL), new device in range(KBluetooth), new buddy online (Kopete) and many more can be configured to do so.
There is however no central way to control the behaviour (when, for how long, where) and appearence of them.
For example it would be nice if they could be stacked in the bottom right corner so that each new is displayed above (in 2D) and once the older is dismissed all newer would fall down.
If you leave your pc for a while, they would not be dismissed but somehow agregated, eg not 4 newmail windows but one saying 4. There should be also a way to cancel a message (device was in range but is gone now = no message)
Link take you to the bugzilla entry.