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If you are developing for Mac OS X or iOS, you should become a registered Apple Developer (http://developer.apple.com/programs/register/). The registration is free. Registering as an Apple Developer allows you to download Xcode and the iPhone SDK. It also allows you access to Mac and iPhone online documentation, sample code, and Apple’s online bug reporting system.
Apple also has two paid programs, the Mac Developer Program (http://developer.apple.com/programs/mac/) and the iPhone Developer Program (http://developer.apple.com/programs/iphone/). Each program costs US$99 per year.