I've just released Notifications, an addon that shows banner-style notifications at the top of the screen, similar to those on modern smartphones. A callback function can be passed as a parameter, which will be executed on click. They are animated too, and have sound! People can use the API function in their addons/UI to implement them - it could be used for anything.
There's a queue system in case multiple notifications come in at the same time. They are also stored when you are AFK, and shown one by one when you get back.
There are options to change the appearance entirely, which can be applied locally or remotely, so that UI developers can include Notifications in their UI and still allow people to update the addon separately without needing to modify any code.
I will include a few usages in my UI, and perhaps release a few example addons.