I'm sure most people know this, but Firefox's developer tools has this feature for any website you're on. Its very cool and sometimes quite handy when developing for web.
"Reveal brings the power of tools like Firebug and Web Inspector to iOS developers."
Symbiote (https://github.com/TestingWithFrank/symbiote) and DCIntrospect (https://github.com/domesticcatsoftware/DCIntrospect) were also inspirations.