There are tons of notifications that get posted by various components in the iOS SDK. The documentation can be found with the class posting the notification, not NSNotificationCenter itself.
For example, you're looking for these: http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIApplication_Class/Reference/Reference.html#//apple_ref/doc/uid/TP40006728-CH3-DontLinkElementID_4