polypane
The link to X on page Y doesn't work is probably a bug report you've received at some point. But how do you find these broken links before someone else does?
In Polypane, the Outline Panel will show you all the links on the page, and check if they return a 200 status code, or if they return an error code (like 404, or 500).
Along with broken URLs, Polypane also finds unfinished/placeholder URLs like mailto: and https:// without anything else after that text.
