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