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.

Unfortunately the main site seems to have gone but the source code is still there. And Internet Archive does have some of the pages

The executable can be got from package manager like Macports - after installing Macports then sudo port install cocoadialog

Homebrew provides the zenity package. It uses the MacOS X11 server (emulation) Xquartz.

Makes it easier to have cross-OS implementations, but it is not native MacOS X gui.

Try also Pashua.

Pashua is a tool for creating native Aqua dialog windows from programming languages that have none or only limi­ted support for graphic user inter­faces on Mac OS X. Currently, it supports Apple­Script, Perl, PHP, Python, Groovy, Rexx, Ruby, shell scripts and Tcl—and if your favourite language is not included in this list: writing the glue code for communicating with Pashua is pretty simple.