Back in the old times, when dinosaurs roamed the earth (and used less sophisticated IDEs), Interface Builder offered a built-in class browser as part of the project window. This browser allowed to you navigate through the Objective-C class hierarchy, and add subclasses along with instance variables and methods. You could generate files from those classes as a skeleton for further development.
Then for a while, the class browser went away. And it was missed. But it is back again. New to Xcode 3.2, the Interface Builder Library pane hosts an updated class browser. This new subpane combines features that have recently been in the Class Identity Inspector (namely, adding outlets and actions to a class) with the ability to generate new subclasses from existing classes.
So how does it work? I may be a brontosaurus but I prefer the old style browser to the new style "Lineage" display shown here. The new pane is certainly pretty, and it fits in well with the Library pane concept of collecting elements that are universally used throughout a project, but it lacks a certain ease-of-browsing that the old tree-style presentation used to give.
All aesthetic and usability concerns aside, it's important to know that the Outlets/Actions interface has moved from its prior home into a new one. The interaction objects remain essentially unchanged. Use the + button to add outlets and actions, the - button to delete them. Double-click the default types to change them to a different class.
You can locate a class by entering a string into the search field at the bottom of the pane. The pop-down action menu on the bottom-left offers a number of class-related functions including subclassing, displaying group banners in the class list, writing out updated class files, and more.