Prometheus - An open-source systems monitoring and alerting toolkit.
ServerSuit - ServerSuit is a browser based program that enables remote Linux administration, monitoring, website hosting, and server setup automation.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
StatsD - Simple daemon for easy stats aggregation.
AppZero - AppZero is a monitoring and migration tool that allows users to keep track of different applications and servers in both simple and complex IT environments.
Hosted Graphite - Graphite as a service