Enterprise-Class Open Source Monitoring
Nagios is the leader in system, network, and application monitoring. It's the industry standard for good reason.
Nagios is an open source monitoring systems that keeps an eye on your network, hosts and even services using plug-ins and SNMP.
It's web-based user interface allow you to quickly discover problems and track their cause - and it also provides easy access to all the information your need about your hosts, etc.
Web and mail are just some of the services Nagios it capable of monitoring - on top of that it will monitor load, memory and disk usage on the hosts running your services.
Nagios can also send notifications through either e-mail, instant messaging or SMS.