One of my favorite open source network and monitoring solutions is Zenoss Core. It is so easy to setup and configure! Just like most monitoring solutions though, you need to make adjustments for you environment to avoid false positives and avoid alerts that you don't care about.
One event that I see a lot from my Xen Servers is "threshold of high utilization exceeded". It happens so often, but the VM's seem to be fine. I decided to raise the limit a bit, but couldn't find where to make that change. Well, after digging I finally found where you can modify that setting!
It's located under Advanced > Monitoring Templates. Then at the bottom you need to select the Device Class button. From there you have two templates you need to modify:
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Obviously that last decimal number represents the percentage of utilization, so increasing it from .75 to .90 changed it from 75% to 90%. Makes sense right?
Anyway, I hope this helps you keep your alerts down to manageable level.