Nagios is a host, service, and network monitoring system that will watch your network and alert you to problems before your clients or end-users do. The system runs checks on hosts and services that you specify using plugins that return status information to Nagios. When problems are encountered, the system will send notifications to system administrators so that they can take action on the problem. The JumpBox for Nagios gives you a head start to using the system. It eliminates the complexity involved in getting the application installed, and allows you to focus on the configuration for your specific environment. Since Nagios is based on plugins, depending on what you want to do this will vary in complexity.
GroundWork Monitor Community Edition can give you insight into your computing infrastructure, allowing you to see the current and historical states of all your computers: servers, desktops, and laptops, all of your network devices, all of your services (like TCP/IP and Web services), and all of your applications (like mail servers and database apps). You can choose to be alerted when something goes awry via pager, SMS, email, or phone, and even set up automatic restarts or fall-overs.
BigDaddy is a program for monitoring servers. It is similar to Nagios, with the added benefit of also monitoring and controlling the crontab (or any scheduled application) across an entire fleet of servers. The application comes in the form of a daemon for monitoring and reporting as well as an easy-to-use Web-based GUI for controlling monitoring, viewing timelines of incidents, filing incidents and graphing statistics. The application is extensible with any sort of monitoring module and notification is based on a five step escalation process.
System Local Audit Daemon can run standalone or managed by systems like IBM-Tivoli, HP-OpenView, or Nessus to perform local security checks. It runs on the target hosts and enables them to call security tools like John the Ripper, Tiger, Tripwire, or a virus scanner via a unified XML interface. It is part of the BOSS Project.