Begun in 1999, OpenNMS an enterprise scale monitoring solution written in Java, with a Postgresql database and configured with XML files. --- Because it uses SNMP OpenNMS can monitor most servers, network gear, and other appliances without needing a custom agent. Its feature list is impressive.
In the coming months I'm going to work with and write about Opennms. Started in 1999, Opennms is a mature, open source, enterprise scale monitoring system.
You want to monitor free disk space.