[cmrdporcupine]

It was an amazing revolution that made complex app development of JS based front ends finally tolerable.

[pjmlp]

IE already had debugging tools, however they had to be installed either via Visual Studio or Office Developer Tools.

[unilynx]

Also they barely worked for anything non trivial, let alone what we now call SPAs

[titusjohnson]

It flat-out made training people on CSS tolerable. I could have them install the Bookmarklet (before it became an Extension), show them how to use it, and boom, instant productivity boost. Being able to dig through the DOM and figure out what element was applying excessive margin or padding reduced 80% of my "heeeelp!" requests.

Within months I couldn't imagine working without it.