Interface Builder was a brilliant program. More than any other it marked the distance between the NeXT development system and that used by any other software company. It helped put NeXTSTEP twenty five years ahead of the pack.
Interface Builder is the work of Jean-Marie Hullot. Hullot originally wrote a program for the Mac called SOS Interface. Steve Jobs saw the program, met Hullot, and hired him on. The rest is real history.
IB stayed fairly stable throughout Jaguar, Panther, and Tiger. Panther introduced Cocoa bindings and so IB invented a new file type: keyedobjects.nib. Unfortunately this file format is grossly inefficient and enhancements to IB and Cocoa were put in the 'keyedobjects.nib' part of the program.
Developers also discovered when upgrading to Tiger that Apple's IB was corrupting a lot of NIBs. Earlier versions didn't notice this but Tiger's IB did. Oops. The NIBs still worked but they were nevertheless corrupted.
NIBs prior to Tiger could end up at practically any size at all. Click ⌘S once and the file size suddenly goes up 4 KB; click it again and it drops down 6 KB. And that's never a good feeling.