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ntAnyone who employs server clusters should have Ganglia monitoring that system. Ganglia can scale to handle clusters with as many as 2,000 nodes.
ntNagios includes a vast number of features. You can even set up email, SMS, and printed paper alerts!
ntZabbix is a powerful network monitoring tool that offers user-defined views, zooming, and mapping on its Web-based console. It also offers agent-less monitoring, collects nearly ANY kind of data you want to monitor, does availability and SLA reporting, and can monitor up to 10,000 devices.