If you’re using a Mac, and you’re not using Growl, you should go grab that right now. I’ll wait until you come back to continue.
Great, now that you have Growl installed, I want to tell you about the Yip Firefox extension. Yip lets you get Growl notifications from websites that have notifications enabled using the Prism and Fluid (those are apps that let you run websites as if they were desktop applications) APIs. I found out about the Yip plugin from the Meebo blog, and Meebo seems like one of the web apps to benefit most from Growl notifications.
So head over to the Yip homepage and install the plugin to see what sites are pushing notifications that you didn’t even know about.