Nagios

This looks really intriguing (at least, if you are concerned with knowing about your server failures before your customers do):

Nagios is a host and service monitor designed to inform you of network problems before your clients, end-users or managers do. It has been designed to run under the Linux operating system, but works fine under most *NIX variants as well.

While it is a Unix-derived monitoring tool, there are companies using it to monitor mixed networks (including Windows-based servers). And based on some of the customer profiles on the web site, Nagios is a serious, Enterprise grade solution.