W3C Link Checker
https://validator.w3.org/checklinkHTML | Tool
The W3C Link Checker crawls your website and reports broken or outdated links. The tool reports on broken links (e.g., 404), broken or missing websites, and links which should be updated to a new location.
Many more W3C checkers exist, for example HTML, CSS, or RSS validation. https://w3c.github.io/developers/tools/ provides an overview.