UptimeRobot - Free Website Uptime Monitoring
nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.