Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
memcached - High-performance, distributed memory object caching system
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.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services