Parasite is a Firebug-like tool for debugging and tinkering with running GTK+ applications, hosted right here on GitHub.

From the page:

You can actually modify your application logic while the program is running! Parasite includes an embedded Python shell that lets you write new code that directly interacts with your application’s UI, regardless of the language the application is written in.

Sounds awesome!

(BTW: the site looks best in Firefox, so try that out if you’re a Safari user.)