UptimeRobot - Free Website Uptime Monitoring
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
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.
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.