Nagios is free, open source host, service and network monitoring services. Nagios provides an extensible framework, that can monitor pretty much anything using plugins.
You can use the watch command to execute a program or shell script periodically, display its output on screen repeatedly. This allows you to watch the program output change over time. By default, the program is run every 2 seconds. This is useful to monitor memory utilization or disk space usage over time without having to look at scrolling output.