Is the inspection free?

Yes, and there is no signup. Enter a URL, get the findings. There is a rate limit of fifteen inspections an hour per visitor, which exists to keep the server from being used as a scanner, not to push anyone toward a paid tier.

There is an optional email step and nothing else. You can ask for a copy of the result by email, and there is a form if you want someone to look at it with you. Both are things you choose to do, and neither one gates the inspection.