GTK+ debugging tool similar to Firebug
Parasite is a debugging and development tool that runs inside your
GTK+ application's process. It can inspect your application, giving
you detailed information on your UI, such as the hierarchy, X window IDs,
widget properties, and more. You can modify properties on the fly in
order to experiment with the look of your UI.
If you've used Firebug, it's sort of like that, but for GTK+ applications.