Whoisology - Connected database of whois records and whois history
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.
WhoisThisDomain - Domain name lookup tool that allows you to get information about domain registration from WHOIS server.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
JSON WHOIS API - Parsed domain WHOIS data from any WHOIS server via RESTful API with metered pay as you pricing.
Better Uptime - We call you when your website goes down