Monit is a free open source utility for managing and monitoring, processes, programs, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.

While focusing on network security monitoring, Bro provides a comprehensive platform for more general network traffic analysis as well.

Jolokia is a JMX-HTTP bridge giving an alternative to JSR-160 connectors. It is an agent based approach with support for many platforms. In addition to basic JMX operations it enhances JMX remoting with unique features like bulk requests and fine grained security policies.

PSI Probe is a community-driven fork of Lambda Probe distributed under the same open-source license (GPLv2). It is intended to replace and extend Tomcat Manager, making it easier to manage and monitor an instance of Apache Tomcat.
Unlike many other server monitoring tools, PSI Probe does not require any changes to your existing apps. It provides all of its features through a web-accessible interface that becomes available simply by deploying it to your server.