Xymon monitors your hosts, your network services, and anything else you configure it to do via extensions. Xymon can periodically generate requests to network services - http, ftp, smtp and so on - and record if the service is responding as expected. You can also monitor local disk utilisation, logfiles and processes through the use of agents installed on the servers. All of the monitoring results are collected by a central server and used to build a set of webpages that show the status of your network, with drill-down functionality to check up on problems. Xymon will also record the history of each monitored item, so you can generate availability reports and check on the incidents that have occurred. Wherever possible data is also stored for trend analysis and presented as graphs, so you can easily track e.g. the response-time of a business-critical web application over time. In case of problems alerts may be sent in the form of e-mails, SMS-messages or pager-alerts so that staff can respond quickly to problems without having to keep watch over the services.
The easiest way to understand what Xymon does is to see it in action and download it here. For a more comprehensive list of tools look here.