Zabbix
Zabbix is free, open source software that can provide real-time monitoring and analysis of thousands of connected devices, from network infrastructure appliances to servers. Zabbix doesn't require agents on monitored components (though there is a Zabbix agent that can be installed), and has community-created templates that can ease the installation for specific purposes.
That "ease" is important, because the learning and installation curves for Zabbix involve steep climbs. Some of that steep grade comes from the sheer number of technologies supported by, and capabilities provided by, Zabbix. The system can ingest data from SNMP, ICMP, Telnet, and SSH sources along with agents on Windows, Solaris, and Linux servers. The configuration and automation can be accomplished with virtually any programming language, making the pre-installation learning curve as shallow as the post-installation curve is steep.
Zabbix has an active community surrounding the system. The company behind Zabbix earns its money from consulting and professional services around the software.
(Image: Zabbix)