I've been using PRTG for several years now. You can try it for free for 10 days and you can keep using it for 10 sensors only for as long as you like. For me the cost was very reasonable considering all that I am able to monitor. If I ever have trouble with a machine, router, switch or even certain services that I monitor, I can look back and see exactly when the problem started. Plus, the alerts have saved my butt several times. As "the" IT guy at our company, it is indispensable. I do have Nagios installed on my VOIP box, but haven't really looked at advanced settings for it. Maybe I need to....