Xcode is the complete toolset for building Mac OS X and iOS applications — and with Xcode 4, the tools have been redesigned to be faster, easier to use, and more helpful than ever before. The Xcode IDE understands your project’s every detail, identifies mistakes in both syntax and logic, and will even fix your code for you. Quite simply, Xcode 4 will help you write better code.

Xcode 4 has a brand new user interface, built upon proven technologies that Apple itself uses to build Mac OS X and iOS, and that have produced over a quarter million Mac OS X and iOS apps.

Homepage: http://developer.apple.com/xcode/index.php