The CoreFoundation Preference Services.
http://developer.apple.com/documentation/CoreFoundation/Conceptual/CFPreferences/CFPreferences.html
This collection of functions lets you do things that the Cocoa NSUserDefaults System doesn’t yet expose (NSUserDefaults is built on top of CFPreferences).
For example, setting individual values in PreferenceSuites that aren’t attached to a single app. Also, setting PreferencesByHost.
See also WorkingWithUserDefaults