One of my passions is automated monitoring and correction of network and server problems. I have the most experience with SiteScope (primarily a commercial website monitoring tool that has branched out to include protocols, application stacks, and whatever custom stuf you want) and Nagios, which is free and open source, and very very configurable.
I would LOVE to form a company implementing these or similar monitoring tools. I've done this a lot at work, and a little bit on the side for a few friends and their companies.
I've been looking around, and it seems that in addition to Nagios, two other contenders are Zenoss and Cacti. I've heard good things about Zenoss, specifically how it is easier to set up than Nagios. I think I may check it out, though I am a fan of Nagios's flexibility.