I've used a fair amount of GUI tookits (including all the ones mentioned by grandparent), as well as written my own. I have to say that I really like Cocoa (par

I've used a fair amount of GUI tookits (including all the ones mentioned by grandparent), as well as written my own. I have to say that I really like Cocoa (particularly UIKit over AppKit) and feel that of everything I've ever worked with, it gets the most right. Of course, you become bound to a specific platform (I've been meaning to give GNUstep a try, not sure how good it is).