Platypus - Realtime server monitoring and reporting
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Monitornator - Server monitoring made simple
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Healthy Servers - Making server monitoring easy
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.