OS X Mountain Lion supports a new system-wide notification service much like iOS’s. A new button to the right of Spotlight in the menubar allows you to show and hide the Notifications sheet that contains all your recent alerts.

If you would like to quickly disable Notifications for some peace and quiet, simply Option-click the Notifications icon. The icon will dim, and you will stop receiving activity alerts until tomorrow. Option-click it again to reenable.

Alternatively, you can also show the Notifications sheet and scroll all the way to the top. A hidden toggle switch can be easily clicked there as well.

To customize your Notifications or whether you receive them for a specific app, go to System Preferences > Notifications.