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.
Whoisology - Connected database of whois records and whois history
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
Better Uptime - We call you when your website goes down
Bulk Whois API - Bulk Whois API is a provider of parsed WHOIS data in JSON with simple API, competitive pricing, top notch support, and more.