I have discovered Nagios last year. Now I’am utilizing nagios now as Software of Monitoring and Fault Management. It’s stable, simple to utilize and extensible.Many developer have created software as livestatus, NagVis and Automap. Support snmp get, ldap queries and http/https requests. You can describe hosts, service and dependencies among these. Nagios sends mail if discovered some problems and reveals the big problem based on hosts and services description (e.g. parent relationship among hosts). Has a OO organization with Class,Object,Attributes and relationship among classes. Try to install and use Nagios.