Under Mavericks, we’ve got a fun new way to use certain notifications (and even interact with them without having to open an app!). How this behaves is partly dependent on what you’ve got configured within System Preferences> Notifications for the programs that this'll work for.
For example, if you’ve got Messages notifications set to appear as Alerts, then you’ll automatically see a handy “Reply” button when one pops up.
You’re not left out in the cold if you prefer banner-style notifications, though—just hover your cursor over the notification, and the button'll appear.
In either case, when you click that button, you can reply right to messages from the notification itself. Oooooo, shiny.
This also works with notifications for new mail (though you’ll be taken into the program if you choose to reply)…
…and for FaceTime calls, where you’ll get options to reply with a message or get reminded later, too.
And yes, that is my shoulder making a brief appearance as I ducked out of the way of the screenshot. Don't expect a gal to be camera-worthy at the crack of noon, OK?