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.
Squat Captcha - The worst captcha ever! Forces you to do 10 squats
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Humans Not Invited - CAPTCHA that filters out humans and only lets programs pass
Better Uptime - We call you when your website goes down
reCAPTCHA - reCAPTCHA is a free security service that protects your websites from spam and abuse.