In Firefox, you hit Ctrl-Shift-I and you get the Inspect mode, which lets you view the element hierarchy, styles in effect for each element, etc.
We have something like that in GtkParasite, but it is shipped separately, it is cumbersome to build, needs a hack to be loaded into an application...
What if GtkParasite were an integral part of GTK+? Then you would just need a "little magic tool" to enable it at runtime. Maybe you install a "Developer tools" extension in Gnome-shell. You click on an "Inspect" button and select a window, or you select "Inspect this window" from the window's menu - the shell sends a client message to that window, GtkParasite catches it, and activates itself.
We have a few really good IDEs, but we hesitate to recommend them: Anjuta, MonoDevelop...
We should recommend them and ensure that they plug well to things like devhelp.
Officially advertise scattered but useful tools
D-bus explorers and profilers, GObject reference count debuggers, what else?