One thing we’ve run into at VMware is that developing GTK+ applications can be hard. Especially when creating and editing UI elements from code (as opposed to Glade), it’s sometimes challenging to debug problems. If a button isn’t showing up the right size, or something is aligned incorrectly, it could be any number of problems.
In order to address this, we’ve started development on a new debugging tool, which we’re calling Parasite. The name comes from the fact that it runs in-process with another application (specifically, as a GTK+ module) and infests it with introspection capabilities.
There’s a lot that can be said about what it does, but the best place to start is probably a screencast.
There’s a lot more information, some screenshots, and instructions for getting the source at http://chipx86.github.com/gtkparasite/