- AppleScript - OAOSAScript makes it super-easy to run AppleScripts from within a Cocoa app, and companion classes introduce features such as automatically populated script menus, scripts on toolbars, and extensions to the Text suite allowing scripters to manipulate rich text.
- OAPreferences (See OmniAppKitPreferences) - An architecture for building multi-pane Preferences windows. All you need to do is write the individual preference panes (which is quite easy) and OAPreferences will automatically generate an appropriate Preferences window ? one like System Prefrences if you have lots of panes, or one like Mail's Preferences window if you only have a few.
- OAFindPanel - Apple has a standardized architecture for Find in applications, but they leave implementing it entirely up to developers. OmniAppKit provides a powerful Find & Replace panel and a protocol for easily making widgets and documents in your app searchable.
- OAInspector & OAInspectorGroup - These classes help you write 'inspector' pallettes. See OmniGraffle for an excellent example of OAInspectorGroup. It does a really cool animated 'ping' between the pallettes when you drag them close to each other.
OmniAppKit also includes lots of extensions to Apple's classes to make creating rich user interfaces much easier.
Here is a list of the extensions and subclasses that have documentation so far on CocoaDev. (In no particular order, feel free to organize!)
If you are thinking about using the OmniFrameworks you should also look at them and see how much of the classes you are going to use OmniBase and OmniFoundation weigh in at around 650K together (yes that is stripped if I am wrong please tell me), i would say OmniAppKit will be around the same. So either be prepared to take a hit in application size or spend quite some time slimming things down in project builder.