Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
Dead Link Checker - Check for broken links on any website
sitebot.ai - The easiest way to monitor that your website is working as it should. Sitebot uses a real browser to test your site and login automatically.
BrokenLinkCheck - Free Broken Link Checker / Online Dead Link Checking Tool - find bad hyperlinks
Ghost Inspector - Easily create automated browser tests for your websites and web apps. Ensure everything works and looks the way it should. No coding required. 14 day free trial!
LinkChecker - LinkChecker is a free, GPL licensed website validator.