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
UptimeRobot - Free Website Uptime Monitoring
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue