I used to use Nagios [nagios.org...] and it wasn't too bad.
Just avoid the classic n00b mistake I've seen done too many times of running the monitoring software dashboard on the same server being monitored.
Ever try to access a monitor dashboard from a server in distress?
Doesn't work! LOL
I ran into a problem running the monitor dashboard from home. It appears the cable company does some of their work at night which sets off all sorts of false alarms for down servers that are perfectly fine.