Using Zenoss to monitor and alert on critical server functions but have not researched other tools yet. Any recommendations for server monitoring/reporting/alerting tools - either freeware or commercially sold products?

Nagios is the obvious choice, I think it's a horrible bag of nuts and bolts and wouldn't ever choose to use it, but it can do pretty much anything adjust as long as your prepared to code it completely yourself.

I'm a huge fan of Splunk though, which could function on a free license if you want. It's uniquely (?) based about a logging backbone. Just log stuff, somehow, and make sense of the data later rather than doing ping probes and the likes.

For a small - mid level commercial product I was always keen on ManageEngien products, but haven't corssed paths with it for about 6 years now.

OpenNMS, hyperic and groundwork all fall much closer to Zenoss, those 4 seem to be broadly interchangeable in most ways.

You could try the backtrack linux distro running off of a flash drive. Since it already comes w/security tools. However I'm not sure on how the configuring the distro to monitor your existing system. But i am pretty sure once researched it will be trivial especially since we are superior linux users lol. But running applications from backtrack could be a possible route to take and may be worth looking into.

backtrack for monitoring?? that's just silly. That's really bad advice, it's utterly utterly not intended for that.