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.
Domainz.io - Premium domains, for not so premium prices
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Newsy - Monetize your un-used domains and turn them into your own content aggregator with 0 code!
Better Uptime - We call you when your website goes down
DomainWatch - Domain Monitoring Made Easy