Apple's Mountain Lion introduced Notification Center to the Mac. This handy feature lets notification-aware apps display pop-up style alerts when something changes, like an incoming email message, Twitter mention, Calendar appointment and more.
Apple notes that you can access Mountain Lion's Notification Center by swiping your trackpad or Magic Trackpad from left to right. Alternatively, you can click the Notification Center icon in your Mac's menu bar (here's a more thorough look at Notification Center options). Those are great, but I use a third option.
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