StatusPage.io - StatusPage.io is the best way for web infrastructure, developer API, and SaaS companies to get set up with their very own status page in minutes. Integrate public metrics and allow your customers to subscribe to be updated automatically.
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.
Trendy 404s - A set of creative templates for your 404s
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
FourZeroFour - Showcase of the best error pages on the web.
Better Uptime - We call you when your website goes down