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.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
whatdevsneed - This is whatdevsneed.
Better Uptime - We call you when your website goes down
SmallDevTools - Handy developer tools with a delightful interface