Colibri provides an alternative to KDE Plasma 4 notifications.
Colibri notifications look lighter and are completely passive: they do not provide any buttons. You may or may not like this.
Since they are completely passive, they smoothly fade away when you mouse over them, allowing you to interact with any window behind them.
They also do not stack on top of each others: if multiple notifications happen, they will be shown one at a time.
Note: I have no plan for now to port Colibri to KDE Plasma 5.
You need kdelibs >= 4.4.
Colibri can be configured from its System Settings module. If you need help follow the setup guide.
Passive notifications for Plasma first appeared as "Ayatana notifications", an alternative notification system I implemented in Kubuntu Karmic as part of my work for Canonical.
Ayatana notifications were implemented as a patch against the systemtray applet. Colibri is based on this work, but does not require patching any applet.
Colibri contains code I wrote for Canonical, but it is not a Canonical product.