Xcode is Apple's development environment for creating applications for the Mac, iPhone, iPod touch, and iPad. It comes free with your Mac, and can be found on your install DVD. It's also available as a download from the Apple developer site.
Even if you're not a developer, you may want to install the Xcode environment. The included editor is handy for any code-related work you might do. The included Plist editor is a fairly good XML editor, although it's geared toward Apple's Plist format.
And once you have Xcode installed, you may get the urge to try your hand at a little bit of programming. Stop by and see David Bolton, About.com Guide to C/C++ C#. He has a beginner tutorial for creating your first iPhone app.