Zabbix

''Zabbix is open source monitoring software for networks and applications. It offers real-time monitoring of thousands of metrics collected from servers, virtual machines, and any other kind of network device. These metrics can help you determine the current health of your IT infrastructure and detect problems with hardware or software components before customers complain. Useful information is stored in a database so you can analyze data over time and improve the quality of provided services, or plan upgrades of your equipment.''

http://www.zabbix.com/community