Zenity is a tool that is used to display GTK dialog boxes from the command line, often from shell scripts. From it's Freshmeat page,
Zenity is a tool that allows you to display Gtk+ dialog boxes from the command line and through shell scripts. It is similar to gdialog, but is intended to be saner. It comes from the same family as dialog, Xdialog, and cdialog, but it surpasses those projects by having a cooler name.
This tool offers a easy way to use Zenity in your scripts with a Ruby-ish syntax. For example, this code:
Zenity::entry(:title => 'Enter some text', :text => 'This is a password dialog.', :arg => ['hide-text'])
gets translated to this bash command:
zenity --entry --title='Enter some text' --text='This is a password dialog.' --hide-text
Look at the COPYING file. (It's the MIT License)