Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
LinkChecker - LinkChecker is a free, GPL licensed website validator.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
Better Uptime - We call you when your website goes down
Checkbot - Checkbot will boost the SEO, speed and security of your site by checking you're following 50+ web best practices. Browser extension with free and paid plans.