Yad - Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons...
Glade - Glade is a RAD tool to enable quick & easy development of user interfaces for the GTK+ toolkit...
wxFormBuilder - wxWidgets is an excellent framework that enables the creation of multi-platform applications with...
NirCmd - NirCmd is a small utility that allows you to do many useful tasks from command-line, without displaying any user interface: change your display settings, turn off your monitor, open the door of your CD-ROM drive, and more...
dialog - Dialog is a handy and feature-rich enterprise multi-device messenger available for server or cloud.
Dialogblocks - DialogBlocks is a cross-platform wxWidgets dialog editor for Windows, Unix and Mac.