The feature, already available to ChromeOS and Windows Dev Channel users, provides a centralised place for viewing and responding to various alerts from services, apps, and extensions.
At present the feature isn’t too fancy; a status bar item has been added to the OS X menubar; an unread notification count (if applicable) appears beside this; and clicking on the item opens the message center.
In keeping with other platforms the status bar icon does not, currently, change colour when unread messages are waiting.
Also, depending on the type of notification sent (though most apps/sites still use standard ‘webkit’ notifications) you may see so-called ‘action buttons’. These range from ‘send message’ on an email notifications to a ‘call’ button on a contact alert.
Google Chrome Canary for Mac OS X can be downloaded at the the following link: