When you get a new badge on stackoverflow.com, a bar appears at the top of the screen informing you of your achievement. It sticks around until the user closes it.
I rather like that system of informing the user about new news related to the site or their account. It's fairly unintrusive, but still clearly communicates the information. Even if all users receive a notification this way, it sticks around for each user until they have acknowledged seeing it.
I'm running a system using Ruby on Rails on a PostGres database. What's the best way to implement a similar system on my setup?
Edit: Just to clarify, I'm interested both in the server side and client side of the setup.