Beats - Beats is the platform for single-purpose data shippers that is installed as lightweight agents and send data to machines to Logstash or Elasticsearch.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Riemann - Container Monitoring
Observium - Observium
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Clerk - Clerk.io, the artificial intelligence for e-commerce that knows your customers interests.