Is there a Mac OSX equivalent of zenity?
Check out cocoaDialog:


  cocoaDialog is an OS X application that allows the use of common GUI controls such as file selectors, text input, progress bars, yes/no confirmations and more with a command-line application. It requires no knowledge of Cocoa, and is ideal for use in shell and Perl scripts (or Ruby, or Python, or... etc).


It's a pretty simple concept — pass arguments to the executable to create a dialog, and it returns a result string. There are some good examples as well as documentation.