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...
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...
VisualWX - Visualwx aims to be the ultimate RAD tool, designed for c++, python, perl, ruby, lua and wxWidgets.
wxDev-C++ - wxDev-C++ is an extension of Bloodshed Dev-C++ by Colin Laplace et. al.