I just started playing with Nagios, an open-source monitoring software package (GPL). I used to use monit instead, but there are two limitations of monit that made me switch: It can only do port/protocol checks on remote hosts It has no tolerance setting for check failures (it sends a warning as soon as there is one failure) On the other hand, Nagios has tools that allows a Nagios server to perform "local" checks on remote servers, via the network (check_snmp, check_nt, check_nrpe and check_ssh). It has as side effect that it can monitor Windows servers quite well. The web interface enough for my needs. Note: I'm still using monit for process checks, as Nagios can't do that as well as monit does (monit uses the information in the lockfile to see if the process is still in memory, and uses user-defined commands to restart the process if it is not in memory). Here is how Nagios works, basically: The tools that do the checks are called plugins Objects have to be defi...