Why these are free and ungated
Every tool in this category asks for your email before it shows you an answer you could have got from a DNS lookup. That is a lead capture form wearing a tool costume, and it wastes the time of the person it is supposed to help.
These do not. They run in your browser, they store nothing, and there is no form. If the result tells you something is broken and you would rather someone else fixed it, the call is there. If not, take the answer and go.
What is coming
I add one when I find myself needing it twice. If there is something you keep doing by hand in outbound that a small tool would solve, tell me on LinkedIn and I will probably build it.