MacZoop ( http://www.maczoop.com ) is an application framework for classic MacOS and Carbon, based on C++, which was designed to be simpler than Metrowerks’ PowerPlant but still full-featured. It is now in “maintenance mode”, i.e. no new features, but it’ll be kept running, and the author suggests using Cocoa if MacZoop doesn’t suffice.
RobRix cut his teeth on it… and then he got ObjC.pdf.
UliKusterer wrote a number of applications with it (e.g. the Talking Moose control panel, and the Resurrection resource editor) and provided a whole set of “views” classes for it (Uli’s Panes), part of the GamePak that added transitions and other game-relevant stuff to MacZoop. And when Cocoa came along, he got Hillegass’ book and switched to Cocoa in one weekend, never looking back.
Footnote: GrahamCox (who wrote most of MacZoop, i.e. me) is also now an active Cocoa developer. Also switched over after a few days reading Hillegass. Interestingly many concepts in Cocoa will be familiar to MacZoop developers, but also a few things in Cocoa are much simpler and once you see them, sort of obvious - like having a central notification centre instead of distributing messages all over the framework. Duh.