Parasite is a program much like Firebug, except this is for GTK based applications.
By opening a program with parasite enabled, you can literally click elements of the GUI to reveal their properties.

The installation is pretty simple, open a terminal, clone the git repo, compile install and run. I like to make a folder for software I checkout from SVN or Git in my home dir so I don’t get things all messy. You need git to clone, so if you don’t have it installed already:
$ sudo apt-get install git-core
Clone it:
$ mkdir ~/git_software && cd ~/git_software && git clone git://github.com/chipx86/gtkparasite
Compile and install it:
$ ./autogen.sh && make && sudo make install
I run 64bit, so I had to do the following command to get things working:
$ sudo cp /usr/local/lib/gtk-2.0 /usr/lib64/ -r
Try it out (prefix an application name with GTK_MODULES=gtkparasite):
$ GTK_MODULES=gtkparasite gedit