The short answer is XCode, which comes on the OS install disk as an optional install (called something like "Developer Tools"). The long answer is that unless

The short answer is XCode, which comes on the OS install disk as an optional install (called something like "Developer Tools"). The long answer is that unless you're porting an existing C++ app, you'll want to learn Cocoa if you want to write GUI apps. It's both the only supported 64-bit API, and what the iPhone uses. There are bindings for Ruby and Python, but the native language is Objective-C. There's also a semi-supported Scheme-like language called [Nu](http://programming.nu/ http://programming.nu/). The [Purple Newfie](http://books.google.com/books?id=ns6tB-uXuxcC&dq=learning+cocoa+with+objective+c&pg=PP1&ots=kBxjE38vkz&sig=R3cFCQq96aPdC8-uRCImtaK0-Uk&hl=en&prev=http://www.google.com/search?client=safari&rls=en-us&q=learning+cocoa+with+objective+c&ie=UTF-8&oe=UTF-8&sa=X&oi=print&ct=title&cad=one-book-with-thumbnail http://books.google.com/books?id=ns6tB-uXuxcC&dq=learnin...) book is pretty good for going the Objective-C route, but quite dated at this point. Once you've worked through the book, you might play around with Core Data (http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/index.html http://developer.apple.com/documentation/Cocoa/Conceptual/Co...) and Cocoa Bindings (http://developer.apple.com/documentation/Cocoa/Conceptual/CocoaBindings/ http://developer.apple.com/documentation/Cocoa/Conceptual/Co...), which the book totally doesn't cover but are very cool for rapid development. And if you're deranged like me you might enjoy just randomly playing around with Interface Builder first to get a feel for what's possible without too much coding.