Zenity - Zenity is a tool that allows you to display GTK dialog boxes in commandline and shell scripts.
Yad - Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons...
Dialogblocks - DialogBlocks is a cross-platform wxWidgets dialog editor for Windows, Unix and Mac.
Glade - Glade is a RAD tool to enable quick & easy development of user interfaces for the GTK+ toolkit...
wxSmith - wxSmith is a ...
VisualWX - Visualwx aims to be the ultimate RAD tool, designed for c++, python, perl, ruby, lua and wxWidgets.