Firebug. Every developer from around 2006-2008 knows what I'm talking about. Debugging JS in IE6 was like trying to build a house blindfolded with both arms tied behind your back. Firebug is when JS went from just a web augmentation toy that could silently fail and your web page would still mostly function to becoming a critical function for a web page (many will see this as all a big mistake).
So not a very good example at all, in fact quite the opposite, it just happened to ride the back of the anti-M$ wave and wasn't at all innovative, copying its features from already existing products.
Conversely, though it was free, IEDevToolbar was only really advertised to MSDN subscribers, so never got much penetration, especially as by then everyone hated IE6 and a lot of Devs worked in Firefox first.
But my company only supported IE6, and when we finally started supporting Firefox, firebug was an annoyingly sub-par experience when debugging js.
Even Chrome's early Dev tools had some sorely missed features when it first came out, as far as I can remember it took them years to support being able to just hold your mouse over a variable and see its value. And because it was a web-page about a web-page you got all these weird bugs. And the CSS editor was (and still is) super annoying with its attempts to cut up your text.
On top of that visual studio also had a JavaScript debugger, I'm sure there were plenty more paid tools you simply didn't know about.