NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
PagerDuty - Cloud based monitoring service
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
xMatters - xMatters transforms event data into intelligent communication IT teams, avoiding outages and disruptions