VMware's Christian Hammond and David Trowbridge have released a new tool called Parasite that can hook into GTK+ applications to facilitate interactive GUI debugging.
Parasite will provide a visual outline of the widget hierarchy and allow you to dynamically modify various widget properties while the program is running. It also includes a built-in Python shell interface that you can use to programmatically manipulate the GUI. You can even use it to add new GTK+ widgets to a window while the program is running. A few other features are also bundled, including a tab that shows GTK+ Action items and a special mode that will display a visual indication when widgets are being repainted.
Read the complete article by Ryan Paul.