Open Source Monitoring Solution

Nagios -Nagios is an Open Source host, service and network monitoring program. http://www.nagios.org/

Groundwork - GroundWork Open Source solution delivers the right solution to ensure unprecedented network, system and application availability. http://www.groundworkopensource.com/

Orca - Orca is a tool useful for plotting arbitrary data from text files onto a directory on a Web server.
http://www.orcaware.com/

Splunk - Splunk is software that securely manages logs and IT data. It is search engine for IT data.
http://www.splunk.org/

Zabbix - ZABBIX offers advanced monitoring, alerting and visualisation. http://www.zabbix.com/

SNMP Informant - SNMP Informant is an innovative and enabling technology that extends and adds value to the SNMP Management of Windows operating systems. It collects low-level, mission critical performance and operational data.
http://www.snmp-informant.com/

Pandora FMS -Pandora watches systems, applications and allows you to know the status of any element of those systems.
http://pandora.sourceforge.net/en/index.php?sec=main

Zenoss- Zenoss CoreTM is an open source, enterprise-grade IT management product that delivers the functionality the IT operations teams need to effectively manage the configuration, health and performance of their networks, servers and applications through a single, integrated software package.Zenoss has First commercial open source CMDB in the market.
http://www.zenoss.com/

Snare - Snare Open Source agents provides a log collection, analysis, reporting and archival resource that is both easy to use and inexpensive.
http://www.intersectalliance.com/

Netdisco - Netdisco is an Open Source web-based network management tool. http://netdisco.org/

OpenSMART - Open (SourceSystem) Monitoring and Reporting Tool. http://opensmart.sourceforge.net/

Pentaho - The Pentaho BI Project provides enterprise-class reporting, analysis, dashboard, data mining and workflow capabilities that help organizations operate more efficiently and effectively.
http://www.pentaho.com/

For more Open Source monitoring tools goto sourceforge.net
http://sourceforge.net/search/?type_of_search=soft&words=monitoring