Using the open source Nagios software, our monitoring system (affectionally referred to as our "watchtower") constantly checks a huge amount of services and functions on all servers. In order to provide the most accurate perception of state we have this machine housed at a third party location so in the event of more large scale problems affecting the network, this does not go unnoticed (which would happen if housed in the same building).
In addition to our own monitoring system we also employ the services of Alertra who provide third party monitoring for all shared/reseller servers at regular intervals. Alertra provide full statistics and uptime percentages for each month which we publish for all to see.

Included features:
They were very good in answering any queries I had before I signed up. They answered promptly and know what they are talking about.