Nagios is server monitoring tool,it is free sotware

Nagios isn't the easiest web-based software solution to install (most of the server configuration is done by editing configuration files), it is extremely easy to use, once configured. Nagios presents you with a web-based status screen, which allows you to quickly view the status of all of the servers that you are monitoring. Nagios isn't for just Linux servers either, Windows servers can be added to monitor as well.