Zabbix - Track, record, alert and visualize performance and availability of IT resources
OctoPerf - OctoPerf is a SaaS load testing solution, based on JMeter.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
StresStimulus - Load testing tool for websites and mobile that works with hard-to-test applications.
Prometheus - An open-source systems monitoring and alerting toolkit.
SmartMeter.io - Performance Testing with Enterprise-Level Features with a familiar jMeter interface