G-Inspector, a tool visualizing internal data structures of Gtk+ application, and Grift, a library which enables separate description between core logic and the user interface in an application software.
a free user interface builder for GTK+ and GNOME, using libglade, Glade XML files can be used in numerous programming languages including C, C++, Java, Perl, Python, C#, Pike, Ruby, Haskell, Objective Caml and Scheme. Adding support for other languages is
a free user interface builder for GTK+ and GNOME interfaces designed are saved as XML, and by using the libglade library these can be loaded by applications dynamically as needed. (Glade can also generate C code)