Checkify - Super minimal, simple and inexpensive 24/7 uptime monitoring
Xenu's Link Sleuth - Xenus Link Sleuth checks Web sites for broken links.
Checkout X - Checkout X is an all-in-one, checkout platform optimized to maximize conversion rates and revenue on WooCommerce stores.
LinkChecker - LinkChecker is a free, GPL licensed website validator.
Curler - Dead Simple Free Uptime Monitoring
Scrutiny - Scrutiny for Mac helps you manage and improve any number of websites. Link check, search, full website spell check, XML sitemap generator, SEO audit