Pandora FMS is a performance & availability monitoring system, ready for big environments. It uses agents for local monitoring and can do several kinds of remote network monitoring (SNMP v3, TCP checks, remote WMI probes...) Agents works on Linux, Windows, AIX, HP-UX, Solaris and BSD systems.
Highly scalable (up to 2000 nodes with one single server), completely web-driven and a multitenant interface. It has a very flexible ACL system and a lot of graphical reports and user-defined control screens.