Glade project is a free user interface builder for GTK+ and GNOME.
The user interfaces designed in Glade 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, though this isn’t recommended for large applications.)
By 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 easy too.
NOTE – you need GTK+ 2.6, and GNOME 2.10 (if you want GNOME support). Also note that interfaces created with Glade 2.10 will need GTK+ 2.6 and GNOME 2.10. So if you want your application to work with older versions of GTK+ and GNOME you need to use an older version of Glade.